Serine phosphorylation of STATs

Interferons: mechanisms of action and clinical applications

PURPOSE OF REVIEW

Interferons are pleiotropic cytokines that exhibit important biologic activities, including antiviral, antitumor, and immunomodulatory effects. These cytokines have found important applications in clinical medicine, including the treatment of certain malignancies. The purpose of this review is to provide an update on basic and clinical research in the interferon field.

RECENT FINDINGS

Significant advances have recently occurred in the field of type I interferon signal transduction. It is well known that the interferons transduce signals via activation of multiple signaling cascades, involving Jak kinases, signal transducer and activator of transcription proteins, Map kinases, and IRS and Crk proteins. Recent evidence indicates that the p38 Map kinase pathway plays an important role in type I interferon signaling in malignant cells and that its function is required for type I interferon-dependent gene transcription and generation of the antiproliferative of type I interferons. In clinical oncology, interferon-alpha remains an active and useful agent in the treatment of several malignant disorders, and efforts are underway to improve its efficacy by using different schedules and combinations with other agents.

SUMMARY

This review summarizes the mechanisms of signal transduction of interferons and the emerging new concepts in this area. An update on the clinical applications of interferons in oncology is also provided, and potential translational applications, reflecting recent advances in the field, are discussed.

Interferon-gamma: an overview of signals, mechanisms and functions

Interferon-gamma (IFN-gamma) coordinates a diverse array of cellular programs through transcriptional regulation of immunologically relevant genes. This article reviews the current understanding of IFN-gamma ligand, receptor, signal transduction, and cellular effects with a focus on macrophage responses and to a lesser extent, responses from other cell types that influence macrophage function during infection. The current model for IFN-gamma signal transduction is discussed, as well as signal regulation and factors conferring signal specificity. Cellular effects of IFN-gamma are described, including up-regulation of pathogen recognition, antigen processing and presentation, the antiviral state, inhibition of cellular proliferation and effects on apoptosis, activation of microbicidal effector functions, immunomodulation, and leukocyte trafficking. In addition, integration of signaling and response with other cytokines and pathogen-associated molecular patterns, such as tumor necrosis factor-alpha, interleukin-4, type I IFNs, and lipopolysaccharide are discussed.

Activated signal transducer and activator of transcription 3 (STAT3) supports the malignant phenotype of human pancreatic cancer

BACKGROUND & AIMS

Constitutive activation of signal transducer and activator of transcription 3 (STAT3) has been implicated in regulation of growth and malignant transformation. We therefore analyzed the expression and biologic significance of STAT3 in human pancreatic cancer cells.

METHODS

Expression and activation of STAT3 were investigated by immunohistochemistry and immunoblotting. Functional inactivation of STAT3 was achieved by stable transfection of dominant-negative STAT3 constructs in 2 pancreatic cancer cell lines and confirmed by electrophoretic mobility shift assay and immunoblotting. Cell proliferation and tumorigenicity were evaluated by cell counting, colony formation in soft agar, and xenotransplantation in nude mice. STAT3-dependent cell cycle distribution was monitored by flow cytometry, immunoprecipitation, immunoblotting, and histone H1 and GST-Rb kinase assays.

RESULTS

Compared with nontransformed human pancreas, activated STAT3 is overexpressed in ductal carcinoma cells but not in ducts from chronic pancreatitis. Constitutive activation was also observed in all human pancreatic cancer cell lines examined. Functional inactivation of STAT3 resulted in significant inhibition of anchorage-dependent and -independent proliferation in vitro and reduced tumor growth in vivo. Cell cycle analysis showed a delay of G(1)/S-phase progression due to inhibition of cyclin-dependent kinase 2 activity based on increased expression of p21(WAF1) in vitro and in vivo. Blocking of the STAT3 upstream activator Janus kinase 2 by tyrphostin also resulted in growth arrest because of delayed G(1)/S-phase progression and increased expression of p21(WAF1).

CONCLUSIONS

On malignant transformation, activated STAT3 promotes cellular proliferation by acceleration of G(1)/S-phase progression and thereby contributes to the malignant phenotype of human pancreatic cancer.

Mycobacterium tuberculosis exerts gene-selective inhibition of transcriptional responses to IFN-gamma without inhibiting STAT1 function

Mycobacterium tuberculosis is a highly successful human pathogen. A major component of this success is the pathogen's ability to avoid eradication by the innate and adaptive immune responses throughout the course of infection. IFN-gamma, a potent activator of the microbicidal activities of macrophages, is essential for control of M. tuberculosis infection, but is unable to stimulate macrophages to kill M. tuberculosis. We have found that infection of the human monocytic cell line, THP-1, resulted in reduced cellular responses to IFN-gamma, manifested as impaired induction of CD64 surface expression and transcription. This defect in transcription occurred despite normal activation of STAT1 in infected macrophages: there was no decrease in STAT1 tyrosine or serine phosphorylation, nuclear translocation, or binding of a minimal IFN-gamma response sequence. Assays of STAT1 function in M. tuberculosis-treated cells also revealed no defect in activation of a minimal gamma-activated sequence construct or STAT1 recruitment to and binding at the endogenous CD64 promoter. In addition, M. tuberculosis did not affect histone acetylation at the CD64 promoter. The inhibition of transcription was gene selective: while transcription of CD64 and class II transactivator were decreased, certain other IFN-gamma-responsive genes either were unaffected or were increased by M. tuberculosis. These results indicate that M. tuberculosis inhibits the response to IFN-gamma by a mechanism distinct from either suppressor of cytokine signaling-1 inhibition of STAT1 phosphorylation or protein inhibitor of activated STAT interference with DNA binding, and indicate that other mechanisms of inhibition of IFN-gamma responses remain to be discovered.

Activation of the p70 S6 kinase and phosphorylation of the 4E-BP1 repressor of mRNA translation by type I interferons

The Type I IFN receptor-generated signals required for initiation of mRNA translation and, ultimately, induction of protein products that mediate IFN responses, remain unknown. We have previously shown that IFNalpha and IFNbeta induce phosphorylation of insulin receptor substrate proteins and downstream engagement of the phosphatidylinositol (PI) 3'-kinase pathway. In the present study we provide evidence for the existence of a Type I IFN-dependent signaling cascade activated downstream of PI 3'-kinase, involving p70 S6 kinase. Our data demonstrate that p70 S6K is rapidly phosphorylated on threonine 421 and serine 424 and is activated during treatment of cells with IFNalpha or IFNbeta. Such activation of p70 S6K is blocked by pharmacological inhibitors of the PI 3'-kinase or the FKBP 12-rapamycin-associated protein/mammalian target of rapamycin (FRAP/mTOR). Consistent with this, the Type I IFN-dependent phosphorylation/activation of p70 S6K is defective in embryonic fibroblasts from mice with targeted disruption of the p85alpha and p85beta subunits of the PI 3'-kinase (p85alpha-/-beta-/-). Treatment of sensitive cell lines with IFNalpha or IFNbeta also results in phosphorylation/inactivation of the 4E-BP-1 repressor of mRNA translation. Such 4E-BP1 phosphorylation is also PI3'-kinase-dependent and rapamycin-sensitive, indicating that the Type I IFN-inducible activation of PI3'-kinase and FRAP/mTOR results in dissociation of 4E-BP1 from the eukaryotic initiation factor-4E (eIF4E) complex. Altogether, our data establish that the Type I IFN receptor-activated PI 3'-kinase pathway mediates activation of the p70 S6 kinase and inactivation of 4E-BP1, to regulate mRNA translation and induction of Type I IFN responses.

Thyrotropin-mediated repression of class II trans-activator expression in thyroid cells: involvement of STAT3 and suppressor of cytokine signaling

It has been suggested that class I and class II MHC are contributing factors for numerous diseases including autoimmune thyroid diseases, type 1 diabetes, rheumatoid arthritis, Alzheimer's disease, and multiple sclerosis. The class II trans-activator (CIITA), which is a non-DNA-binding regulator of class II MHC transcription, regulates the constitutive and inducible expression of the class I and class II genes. FRTL-5 thyroid cells incubated in the presence of IFN-gamma have a significantly higher level of cell surface rat MHC class II RTI.B. However, the IFN-gamma-induced RT1.B expression was suppressed significantly in cells incubated in the presence of thyrotropin. Thyrotropin (TSH) represses IFN-gamma-induced CIITA expression by inhibiting type IV CIITA promoter activity through the suppression of STAT1 activation and IFN regulatory factor 1 induction. This study found that TSH induces transcriptional activation of the STAT3 gene through the phosphorylation of STAT3 and CREB activation. TSH induces SOCS-1 and SOCS-3, and TSH-mediated SOCS-3 induction was dependent on STAT3. The cell line stably expressing the wild-type STAT3 showed a higher CIITA induction in response to IFN-gamma and also exhibited TSH repression of the IFN-gamma-mediated induction of CIITA. However, TSH repression of the IFN-gamma-induced CIITA expression was not observed in FRTL-5 thyroid cells, which stably expresses the dominant negative forms of STAT3, STAT3-Y705F, and STAT3-S727A. This report suggests that TSH is also engaged in immunomodulation through signal cross-talk with the cytokines in thyroid cells.

Serine phosphorylation of STAT3 is essential for Mcl-1 expression and macrophage survival

The Bcl-2 family member Mcl-1 is essential for macrophage survival. However, the mechanisms that contribute to the expression of Mcl-1 in these cells have not been fully characterized. The present study focused on the role of signal transducer and activator of transcription 3 (STAT3) in regulation of Mcl-1 in macrophages. Sodium salicylate (NaSal) treatment induced apoptotic cell death in primary human macrophages in a dose- and time-dependent fashion. Incubation with NaSal resulted in the loss of mitochondrial transmembrane potential, the release of cytochrome c and second mitochondria-derived activator of caspase/direct IAP binding protein with low pH of isoelectric point (pI) from the mitochondria, and the activation of caspases 9 and 3. Western blot analysis and reverse transcription-polymerase chain reaction demonstrated that NaSal down-regulated the expression of Mcl-1. Electrophoretic mobility shift assay and Western blot analysis for phosphorylated STAT3 demonstrated that STAT3 was constitutively activated in macrophages and that this STAT3 activation was suppressed by NaSal. The activation of STAT3 in macrophages was dependent on Ser727 phosphorylation, in the absence of detectable Tyr705 phosphorylation. Ectopic expression of STAT3 in murine RAW264.7 macrophages rescued the inhibition of Mcl-1 promoter-reporter gene activation and the cell death induced by NaSal treatment, while a dominant-negative STAT3 resulted in cell death. To confirm its role in primary macrophages, STAT3 antisense (AS) oligodeoxynucleotides (ODNs) were employed. STAT3 AS, but not control, ODNs decreased STAT3 and Mcl-1 expression and resulted in macrophage apoptosis. These observations demonstrate that the STAT3-mediated expression of Mcl-1 is essential for the survival of primary human in vitro differentiated macrophages.

Activation of protein kinase C delta by IFN-gamma

Engagement of the type II IFN (IFN-gamma) receptor results in activation of the Janus kinase-Stat pathway and induction of gene transcription via IFN-gamma-activated site (GAS) elements in the promoters of IFN-gamma-inducible genes. An important event in IFN-gamma-dependent gene transcription is phosphorylation of Stat1 on Ser(727), which is regulated by a kinase activated downstream of the phosphatidylinositol 3'-kinase. Here we provide evidence that a member of the protein kinase C (PKC) family of proteins is activated downstream of the phosphatidylinositol 3'-kinase and is engaged in IFN-gamma signaling. Our data demonstrate that PKCdelta is rapidly phosphorylated during engagement of the type II IFNR and its kinase domain is induced. Subsequently, the activated PKCdelta associates with a member of the Stat family of proteins, Stat1, which acts as a substrate for its kinase activity and undergoes phosphorylation on Ser(727). Inhibition of PKCdelta activity diminishes phosphorylation of Stat1 on Ser(727) and IFN-gamma-dependent transcriptional regulation via IFN-gamma-activated site elements, without affecting the phosphorylation of the protein on Tyr(701). Thus, PKCdelta is activated during engagement of the IFN-gamma receptor and plays an important role in IFN-gamma signaling by mediating serine phosphorylation of Stat1 and facilitating transcription of IFN-gamma-stimulated genes.

EGF receptor-mediated, c-Src-dependent, activation of Stat5b is downregulated in mitogenically responsive hepatocytes

Signal transducer and activator of transcription (STAT) proteins may be activated by epidermal growth factor (EGF), but their role in EGF receptor-mediated mitogenic signaling is not clear. We previously showed that Stat5b was activated by EGF in rat hepatocytes in primary monolayer culture. In the present study, we found that EGF induced tyrosine phosphorylation of Stat5b both on Tyr-699, which correlated with specific DNA binding activity, and also on other tyrosine residues. The Src tyrosine kinase inhibitor CGP77675 blocked the EGF-induced activation of Stat5b, but did not affect the Stat5b activation by growth hormone (GH) or prolactin (PRL). The Stat5b response to EGF was most pronounced soon (3 h) after plating (early G(1)) and at high cell density (50,000 hepatocytes per cm(2)). However, at this cell density EGF did not stimulate DNA synthesis. In hepatocytes at 24 h of culturing (mid/late G(1)) with 20,000 cells per cm(2), EGF induced strong phosphorylation of the EGF receptor, as well as Shc and ERK, and stimulated DNA synthesis, but did not activate Stat5b, although the Stat5b response to GH or PRL was retained. A strong GH-induced Stat5b activation neither influenced the DNA synthesis alone nor enhanced the mitogenic effect of EGF. The results show that EGF induces tyrosine phosphorylation and DNA-binding activity of Stat5b in a manner different from GH and PRL, apparently by a Src-dependent mechanism. The data also provide further evidence that Stat5b is not required for mitogenic signaling from the EGF receptor.

Toll-like receptors 2 and 4 activate STAT1 serine phosphorylation by distinct mechanisms in macrophages

Engagement of Toll-like receptor (TLR) proteins activates multiple signal transduction pathways. These studies show that engagement of TLR2 and TLR4 leads to rapid phosphorylation of the transcription factor STAT1 at serine 727 (Ser-727 STAT1) in murine macrophages. Only TLR4 engagement induced STAT1 phosphorylation at tyrosine 701, although this response was delayed compared with Ser-727 STAT1 phosphorylation. Inhibition of phosphatidylinositol 3'-kinase using LY294002 blocked TLR4-induced STAT1 tyrosine phosphorylation, but this inhibitor had no effect on STAT1 serine phosphorylation. TLR-induced phosphorylation of Ser-727 STAT1 could be blocked by the selective p38 mitogen-activated protein kinase inhibitor SB203580. However, activation of p38 was not sufficient to induce Ser-727 STAT1 phosphorylation in macrophages. TLR2-induced activation of Ser-727 STAT1 phosphorylation required the adapter protein MyD88, whereas TLR4-induced activation of Ser-727 STAT1 phosphorylation was not solely dependent on MyD88. Lastly, TLR4-induced activation of Ser-727 STAT1 phosphorylation could be blocked by rottlerin, a specific inhibitor of protein kinase C-delta. In contrast, rottlerin had no effect on STAT1 phosphorylation induced via TLR2. Together, these data demonstrate that activation STAT1 tyrosine and serine phosphorylation are distinct consequences of TLR engagement in murine macrophages. Furthermore, p38 mitogen-activated protein kinase, protein kinase C-delta, and a novel TLR2-specific signaling pathway appear to be necessary to induce Ser-727 STAT1 phosphorylation.

The p38 mitogen-activated protein kinase pathway and its role in interferon signaling

Interferons (IFNs) are pleiotropic cytokines that exhibit multiple biological effects on cells and tissues. IFN receptors are expressed widely in mammalian cells and virtually all different cell types express them on their surface. The Type I IFN receptor has a multichain structure, composed of at least two distinct receptor subunits, IFNalphaR1 and IFNalphaR2. Two Jak-kinases, Tyk-2 and Jak-1, associate with the different receptor subunits and are activated in response to IFNalpha or IFNbeta to regulate engagement of multiple downstream signaling cascades. These include the Stat-pathway, whose function is essential for transcriptional activation of IFN-sensitive genes, and the insulin receptor substrate pathway, which regulates downstream activation of the phosphatidyl-inositol-3' kinase. Members of the Map family of kinases are also activated by the Type I IFN receptor and participate in the generation of IFN signals. The p38 Map kinase pathway appears to play a very important role in the induction of IFN responses. p38 is rapidly activated during engagement of the Type I IFN receptor, and such an activation is regulated by the small G-protein Rac1, which functions as its upstream effector in a tyrosine kinase-dependent manner. The activated form of p38 regulates downstream activation of other serine kinases, notably MapKapK-2 and MapKapK-3, indicating the existence of Type I IFN-dependent signaling cascades activated downstream of p38. Extensive studies have shown that p38 plays a critical role in Type I IFN-dependent transcriptional regulation, without modifying activation of the Stat-pathway. It is now well established that the function of p38 is essential for gene transcription via ISRE or GAS elements, but has no effects on the phosphorylation of Stat-proteins, the formation of Stat-complexes, and their binding to the promoters of IFN-sensitive genes. As Type I IFNs regulate gene expression for proteins with antiviral properties, it is not surprising that pharmacological inhibition of the p38 pathway blocks induction of IFNalpha-antiviral responses. In addition, pharmacological inhibition of p38 abrogates the suppressive effects of Type I IFNs on normal human hematopoietic progenitors, indicating a critical role for this signaling cascade in the induction of the regulatory effects of Type I IFNs on hematopoiesis. p38 is also activated during IFNalpha-treatment of primary leukemia cells from patients with chronic myelogenous leukemia. Such activation is required for IFNalpha-dependent suppression of leukemic cell progenitor growth, indicating that this pathway plays a critical role in the induction of the antileukemic effects of IFNalpha.

Erythropoietin-induced serine 727 phosphorylation of STAT3 in erythroid cells is mediated by a MEK-, ERK-, and MSK1-dependent pathway

OBJECTIVE

Erythropoietin (EPO) is a key regulator of erythropoiesis, playing a role in both the proliferation and differentiation of erythroid cells. One of the signal transduction molecules activated upon EPO stimulation is signal transducer and activator of transcription (STAT) 3. Besides tyrosine 705 phosphorylation of STAT3, serine 727 phosphorylation has been described upon EPO stimulation. In the present study, we investigated which molecular pathways mediate the STAT3 serine 727 phosphorylation and the functional implications of this phosphorylation.

METHODS

The EPO-dependent erythroid cell line ASE2 was used to investigate which signaling routes were involved in the STAT3 serine 727 phosphorylation. Western blotting using phosphospecific antibodies was used to assess the phosphorylation status of STAT3 molecules. Transfection analysis was performed to investigate the transactivational potential of STAT3, and quantitative RT-PCR was used to study the in vivo gene expression of STAT3-responsive genes.

RESULTS

Western blotting of extracts of cells exposed to various chemical inhibitors revealed that the MEK inhibitors PD98059 and U0126 abrogated the EPO-mediated STAT3 serine 727 phosphorylation without an effect on tyrosine phosphorylation. Further analysis showed that MSK1 is activated downstream of ERK, and retroviral transductions with kinase-inactive MSK1 revealed that MSK1 is necessary for STAT3 serine phosphorylation. Furthermore, the STAT3-mediated transactivation was reduced by blocking the STAT3 serine phosphorylation with the MEK inhibitor U0126 or by expression of kinase-inactive MSK1.

CONCLUSIONS

The EPO-induced STAT3 serine 727 phosphorylation is mediated by a pathway involving MEK, ERK, and MSK1. Furthermore, serine phosphorylation of STAT3 augments the transactivational potential of STAT3.

Transcriptional activation of the human inducible nitric-oxide synthase promoter by Kruppel-like factor 6

Nitric oxide is a ubiquitous free radical that plays a key role in a broad spectrum of signaling pathways in physiological and pathophysiological processes. We have explored the transcriptional regulation of inducible nitric-oxide synthase (iNOS) by Krüppel-like factor 6 (KLF6), an Sp1-like zinc finger transcription factor. Study of serial deletion constructs of the iNOS promoter revealed that the proximal 0.63-kb region can support a 3-6-fold reporter activity similar to that of the full-length 16-kb promoter. Within the 0.63-kb region, we identified two CACCC sites (-164 to -168 and -261 to -265) that bound KLF6 in both electrophoretic mobility shift and chromatin immunoprecipitation assays. Mutation of both these sites abrogated the KLF6-induced enhancement of the 0.63-kb iNOS promoter activity. The binding of KLF6 to the iNOS promoter was significantly increased in Jurkat cells, primary T lymphocytes, and COS-7 cells subjected to NaCN-induced hypoxia, heat shock, serum starvation, and phorbol 12-myristate 13-acetate/ ionophore stimulation. Furthermore, in KLF6-transfected and NaCN-treated COS-7 cells, there was a 3-4-fold increase in the expression of the endogenous iNOS mRNA and protein that correlated with increased production of nitric oxide. These findings indicate that KLF6 is a potential transactivator of the human iNOS promoter in diverse pathophysiological conditions.

Determination of leptin signaling pathways in human and murine keratinocytes

Recently, we have determined the role of leptin as a keratinocyte mitogen in vitro and during skin repair in vivo. In this study, we assessed leptin-stimulated signal transduction in the human keratinocyte cell line HaCaT and the murine keratinocyte cell line PAM 212. HaCaT keratinocytes were characterized by a constitutive phosphorylation of janus kinase (JAK)-2. By contrast, PAM 212 keratinocytes responded to leptin with a rapid phosphorylation of JAK-2. However, we could determine a cytoplasmic activation of signal transducer and activator of transcription (STAT)-3 by phosphorylation of tyrosine 705 (Y705) within minutes only upon leptin stimulation in both keratinocyte cell lines. Subsequently, STAT-3 translocated to the nucleus where serine 727 (S727) was phosphorylated, establishing a transcriptionally active STAT-3 transcription factor. In a model of cutaneous wound healing, treatment of leptin-deficient obese/obese (ob/ob) mice strongly augmented phosphorylation of STAT-3 (Y705) in wound keratinocytes also in vivo.

Signal transducer and activator of transcription proteins in leukemias

Signal transducer and activator of transcription (STAT) proteins are a 7-member family of cytoplasmic transcription factors that contribute to signal transduction by cytokines, hormones, and growth factors. STAT proteins control fundamental cellular processes, including survival, proliferation, and differentiation. Given the critical roles of STAT proteins, it was hypothesized that inappropriate or aberrant activation of STATs might contribute to cellular transformation and, in particular, leukemogenesis. Constitutive activation of mutated STAT3 has in fact been demonstrated to result in transformation. STAT activation has been extensively studied in leukemias, and mechanisms of STAT activation and the potential role of STAT signaling in leukemogenesis are the focus of this review. A better understanding of mechanisms of dysregulation of STAT signaling pathways may serve as a basis for designing novel therapeutic strategies that target these pathways in leukemia cells.

Ataxia telangiectasia mutated proteins, MAPKs, and RSK2 are involved in the phosphorylation of STAT3

Phosphorylation at Ser(727) is known to be required for complete activation of STAT3 by diverse stimuli including UV irradiation, but the kinase(s) responsible for phosphorylating STAT3 (Ser(727)) is still not well discerned. In the present study, we observed that activation of ATM is required for a UVA-stimulated increase in Ser(727) phosphorylation of STAT3 as well as in activation and phosphorylation of p90 ribosomal protein S6 kinases (RSKs). Moreover, UVA-stimulated activation of upstream kinases, such as c-Jun N-terminal kinases (JNKs) and ERKs, involved in mediating phosphorylation of RSKs and STAT3 was defective or delayed in ATM-deficient cells. Furthermore, we provide evidence that RSK2-deficient cells were defective for UV-induced Ser(727) phosphorylation of STAT3, and the defect was restored after ectopic expression of transfected full-length RSK2. In vitro experiments showed that active RSK2 and JNK1 induce the phosphorylation of STAT3 precipitates from immunoprecipitation but not from glutathione S-transferase (GST) pull-down. Interestingly, the GST fusion STAT3 proteins mixed together with STAT3 immunoprecipitates can be phosphorylated by JNK. However, the in vitro phosphorylation of STAT3 was reduced by the GST-STAT3 beta protein, a dominant negative form of STAT3. Taken together, our results demonstrate that the STAT3 phosphorylation at Ser(727) is triggered by active RSK2 or JNK1 in the presence of a downstream kinase or a cofactor, and thereby the intracellular phosphorylation process is stimulated through a signaling pathway involving ATM, MAPKs, RSK2, and an as yet unidentified kinase or cofactor. Additionally, RSK2-mediated phosphorylation of STAT3 (Ser(727)) was further determined to be required for basal and UVA-stimulated STAT3 transcriptional activities.

Phosphorylation of the Stat1 transactivating domain is required for the response to type I interferons

Stat1 (signal transducer and activator of transcription 1) regulates transcription in response to the type I interferons IFN-alpha and IFN-beta, either in its dimerized form or as a subunit of the interferon-stimulated gene factor 3 (Isgf3) complex (consisting of Stat1, Stat2 and interferon-regulating factor 9). Full-length Stat1-alpha and the splice variant Stat1-beta, which lacks the carboxyl terminus and the Ser727 phosphorylation site, are found in all cell types. IFN-induced phosphorylation of Stat1-alpha on Ser727 occurs in the absence of the candidate kinase, protein kinase C-delta. When expressed in Stat1-deficient cells, Stat1-beta and a Stat1-S727A mutant both restored the formation of Stat1 dimers and of the Isgf3 complex on treatment with IFN-beta. By contrast, only Stat1-alpha restored the ability of IFN-beta to induce high levels of transcription from target genes of Stat1 dimers and Isgf3 and to induce an antiviral state. Our data suggest an important contribution of the Stat1 C terminus and its phosphorylation at Ser727 to the transcriptional activities of the Stat1 dimer and the Isgf3 complex.

IRAK-dependent phosphorylation of Stat1 on serine 727 in response to interleukin-1 and effects on gene expression

Interleukin-1 (IL-1) induces the phosphorylation of Stat1 on serine 727 but not on tyrosine 701. Analyses of mutant I1A cells, which lack the IL-1 receptor-associated kinase (IRAK), and of I1A cells reconstituted with deletion mutants of IRAK show that the IL-1-mediated phosphorylation of Stat1 on serine requires the IRAK protein but not its kinase activity and does not involve phosphatidylinositol-3'-kinase (PI3K) or the mitogen-activated protein (MAP) kinases p38 or ERK. IRAK and Stat1 interact in vivo, and this interaction is increased in response to IL-1, suggesting that IRAK may serve to recruit the as yet unknown IL-1-induced Stat1 serine kinase. Chemical inhibitors or dominant-negative forms of signaling components required to activate NF-kappa B, ATF, or AP-1 in response to IL-1 do not affect the phosphorylation of Stat1 on serine. IL-1 and tumor necrosis factor (TNF) enhance the serine phosphorylation of Stat1 that occurs in response to interferon-gamma (IFN-gamma) and potentiate IFN-gamma-mediated, Stat1-driven gene expression, thus contributing to the synergistic activities of these proinflammatory cytokines.

Two nonoverlapping domains on the Norwalk virus open reading frame 3 (ORF3) protein are involved in the formation of the phosphorylated 35K protein and in ORF3-capsid protein interactions

Expression of the Norwalk virus open reading frame 3 (ORF3) in Spodoptera frugiperda (Sf9) cells yields two major forms, the predicted 23,000-molecular-weight (23K) form and a larger 35K form. The 23K form is able to interact with the ORF2 capsid protein and be incorporated into virus-like particles. In this paper, we provide mass spectrometry evidence that both the 23K and 35K forms are composed only of the ORF3 protein. Two-dimensional gel electrophoresis and phosphatase treatment showed that the 35K form results solely from phosphorylation and that the 35K band is composed of several different phosphorylated forms with distinct isoelectric points. Furthermore, we analyzed deletion and point mutants of the ORF3 protein. Mutants that lacked the C-terminal 33 amino acids (ORF3(1-179), ORF3(1-152), and ORF3(1-107)) no longer produced the 35K form. An N-terminal truncation mutant (ORF3(51-212)) and a site-directed mutant (ORF3(T201V)) were capable of producing the larger form, which was converted to the smaller form by treatment with protein phosphatase. These data suggest that the region between amino acids 180 and 212 is phosphorylated, and mass spectrometry showed that amino acids Arg196 to Arg211 are not phosphorylated; thus, phosphorylation of the serine-threonine-rich region from Thr181 to Ser193 must be involved in the generation of the 35K form. Studies of the interaction between the ORF2 protein and full-length and mutated ORF3 proteins showed that the full-length ORF3 protein (ORF3(FL)), ORF3(1-179), ORF3(1-152), and ORF3(51-212) interacted with the ORF2 protein, while an ORF3(1-107) protein did not. These results indicate that the region of the ORF3 protein between amino acids 108 and 152 is responsible for interaction with the ORF2 protein.

A new trick for Cyclin-Cdk: activation of STAT

New work in Drosophila demonstrates that cdk4 loss causes phenotypes similar to the loss of JAK/STAT pathway components. Cdk4 overexpression can bypass requirements for JAK but not STAT. These results demonstrate a new function for Cdk4 and a new mode of STAT activation.

Rho family GTPases are required for activation of Jak/STAT signaling by G protein-coupled receptors

As do cytokine receptors and receptor tyrosine kinases, G protein-coupled receptors (GPCRs) signal to Janus kinases (Jaks) and signal transducers and activators of transcription (STATs). However, the early biochemical events linking GPCRs to this signaling pathway have been unclear. Here we show that GPCR-stimulated Rac activity and the subsequent generation of reactive oxygen species are necessary for activating tyrosine phosphorylation of Jaks and STAT-dependent transcription. The requirement for Rac activity can be overcome by addition of hydrogen peroxide. Expression of activated mutants of Rac1 is sufficient to activate Jak2 and STAT-dependent transcription, and the activation of Jak2 correlates with the ability of Rac1 to bind to NADPH oxidase subunit p67(phox). We further show that GPCR agonists stimulate tyrosine phosphorylation of STAT1 and STAT3 proteins in a Rac-dependent manner. The tyrosine phosphorylation of STAT3 is biphasic; the first peak of phosphorylation is weak and correlates with rapid activation of Jaks by GPCRs, whereas the second peak is stronger and requires the synthesis of an autocrine factor. Rho also plays an essential role in the induction of STAT transcriptional activity. Our results highlight a novel role for Rho GTPases in mediating the regulatory effects of GPCRs on STAT-dependent gene expression.

The role of prolactin in mammary carcinoma

The contribution of prolactin (PRL) to the pathogenesis and progression of human breast cancer at the cellular, transgenic, and epidemiological levels is increasingly appreciated. Acting at the endocrine and autocrine/paracrine levels, PRL functions to stimulate the growth and motility of human breast cancer cells. The actions of this ligand are mediated by at least six recognized PRL receptor isoforms found on, or secreted by, human breast epithelium. The PRL/PRL receptor complex associates with and activates several signaling networks that are shared with other members of the cytokine receptor superfamily. Coupled with the recently identified intranuclear function of PRL, these networks are integrated into the in vitro and in vivo actions induced by ligand. These findings indicate that antagonists of PRL/PRL receptor interaction or PRL receptor-associated signal transduction may be of considerable utility in the treatment of human breast cancer.

Proteomics and systems biology approaches to signal transduction in sepsis

Sepsis and resulting multiple system organ failure are the leading causes of mortality in intensive care units. Although it is generally appreciated that rampant, deregulated inflammatory pathways play a major role in sepsis, a comprehensive understanding based on the integrated response of multiple signal transduction pathways has remained elusive. Here we review the main signal transduction pathways involved in the progression from inflammation to sepsis and discuss emerging genomic, proteomic, and systems biology approaches to decipher how these signaling pathways converge to cause the septic state. We propose that an integrative approach involving functional proteomics will provide a quantitative and mechanistic description that unifies inflammatory signaling networks in sepsis and will identify critical regulatory nodes for therapeutic manipulation. These types of systems biology-based approaches may lead to more effective therapies than those currently available.

STAT5a activation mediates the epithelial to mesenchymal transition induced by oncogenic RhoA

The involvement of Rho GTPases in signal transduction pathways leading to transcription activation is one of the major roles of this family of GTPases. Thus, the identification of transcription factors regulated by Rho GTPases and the understanding of the mechanisms of their activation and its biological outcome are of great interest. Here, we provide evidence that Rho GTPases modulate Stat5a, a transcription factor of the family of signal transducers and activators of transcription. RhoA triggers tyrosine phosphorylation (Y696) of Stat5a via a JAK2-dependent mechanism and promotes DNA-binding activity of Stat5a. Tyrosine phosphorylation of Stat5a is also stimulated physiologically by lysophosphatidic acid (LPA) in a Rho-dependent manner. Simultaneously, RhoA reduces serine phosphorylation of Stat5a at both serine residues S726 and S780, resulting in a further increase of activity as defined by mutagenesis experiments. Furthermore, serine dephosphorylation of Stat5a by RhoA does not take place by down-modulation of either JNK1, MEK1, or p38 MAP kinases, as determined by transfection experiments or chemical inhibition of both MEK1, p38, and JNK serine kinases. Thus, RhoA regulates Stat5a via tyrosine phosphorylation and via a yet to be determined novel down-modulating pathway that involves serine dephosphorylation. Finally, we provide evidence for a role of Stat5a in RhoA-induced epithelial-to-mesenchymal transition with concomitant increase in vimentin expression, E-cadherin down-regulation, and cell motility.

SHP2 regulates IL-2 induced MAPK activation, but not Stat3 or Stat5 tyrosine phosphorylation, in cutaneous T cell lymphoma cells

The phosphotyrosine phosphatase SHP2 has been suggested to regulate activation of MAPK, Stat3, and Stat5 in several experimental models. In this study we investigated the role of SHP2 in IL-2 induced activation of MAPK and the Stat proteins using the human CTCL cell line MyLa2059 derived from a cutaneous T cell lymphoma (CTCL). For this purpose, MyLa2059 cells were stably transfected with wild-type SHP2 or inactive SHP2. The cells transfected with inactive SHP2 showed reduced MAPK activation upon IL-2 stimulation, suggesting that SHP2 upregulates IL-2 induced MAPK activation in T cells. However, the constitutive tyrosine phosphorylation of Stat3 as well as IL-2 induced Stat5 tyrosine phosphorylation and DNA binding were unaffected by the stably transfected wild-type SHP2 as well as the inactive SHP2. In conclusion, we show for the first time that SHP2 positively regulates IL-2 induced MAPK activation in malignant T cells. Furthermore, the results indicate that SHP2 may not be involved in the activation of Stat3 or Stat5 in CTCL cells.

STAT1 is activated in neurons after ischemia and contributes to ischemic brain injury

Signal transducers and activators of transcription (STAT) proteins are a family of transcription factors that play a crucial role in growth and differentiation in a variety of cell types. Among them, STAT1, which is expressed in the brain and directly activated by reactive oxygen species, participates in the regulation of cytokine-signaling and cellular responses, particularly to interferon-gamma. Very little, however, is known about the importance of STAT1 during brain injury. The authors found that STAT1 was phosphorylated at tyrosine and serine727 and translocated into neuronal nuclei within hours after middle cerebral artery occlusion. At later time points, STAT1 immunoreactivity colocalized with TUNEL-positive neurons, thereby suggesting a role in cell death. In mice genetically deficient in STAT1 expression, the volume of ischemic brain injury was reduced, neurologic deficits were less severe, and TUNEL-positive neurons were also less numerous compared with wild-type mice. STAT1-knockout mice showed increased phosphorylated Akt and decreased procaspase-3 cleavage. Major strain differences in phosphorylated STAT3 or cyclooxygenase-2 protein expression were not found after ischemia. These results indicate that STAT1 is activated and translocated within ischemic neurons and may contribute to brain injury by regulating transcription and phosphorylation of proteins related to apoptosis and cell death.

p38 Mitogen-activated protein kinase regulates interleukin-4-induced gene expression by stimulating STAT6-mediated transcription

STAT6 functions as a critical mediator of IL-4-stimulated gene activation, and the function of STAT6 is regulated by both tyrosine and serine kinase activities. Here we analyzed the role of serine phosphorylation in regulation of STAT6-mediated transcription. Optimal transcriptional response of IL-4-inducible promoters requires costimulatory signals through CD40-stimulated intracellular kinases such as p38 MAPK. We found that the p38 MAPK inhibitor SB202190 as well as the dominant negative p38 MAPK inhibited interleukin (IL)-4 regulated expression of CD23 in Ramos B cells. IL-4 stimulation did not stimulate p38 MAPK activity, but inhibition of p38 MAPK activity directly correlated with inhibition of IL-4-induced gene activation. Dissection of individual response elements on IL-4-regulated promoter showed that C/EBP beta-mediated transcription was insensitive to SB202190 treatment in B cells whereas STAT6-mediated transcription was regulated by p38 MAPK. The IL-4-induced immediate activation events of STAT6 were not affected by p38 MAPK activity. Furthermore, phosphoamino acid analysis and phosphopeptide mapping indicated that STAT6 is not a direct substrate for p38 MAPK. Instead, p38 MAPK was found to directly regulate the activity of the transactivation domain of STAT6. These results show that, in addition to the well established proinflammatory effects, p38 MAPK also provides a costimulatory signal for IL-4-induced gene responses by directly stimulating the transcriptional activation of STAT6.

Signal transducers and activators of transcription as downstream targets of nongenomic estrogen receptor actions

17Beta-estradiol-activated estrogen receptor alpha (ERalpha) and beta (ERbeta) are able to induce transcriptional activation of signal transducer and activator of transcription (Stat)-regulated promoters via cytoplasmic signal transduction pathways. Stat5 and Stat3 are required for promoter induction, which correlates with cytoplasmic sublocalization of ERs and is independent of intact coactivator binding sites and DNA-binding domains. In endothelial cells, Stat5 and Stat3 are rapidly phosphorylated on both tyrosine and serine residues in response to 17beta-estradiol, and nuclear translocation is subsequently induced. 17Beta-estradiol-induced transactivation of a Stat-regulated promoter requires at least three different signal transduction pathways, including MAPK, Src-kinase, and phosphatidylinositol-3-kinase activities. In conclusion, this work identifies a novel pathway involving an agonist-bound ER-activated phosphorylation cascade, resulting in nuclear transcriptional activation of target transcription factors. These findings reveal novel targets for the development of drugs that modulate a nongenomic-to-genomic ER-dependent mechanism.

Opposing cytokine-specific effects of all trans-retinoic acid on the activation and expression of signal transducer and activator of transcription (STAT)-1 in THP-1 cells

The regulation of signal transducer and activator of transcription-1 (STAT-1) by cytokines and all-trans-retinoic acid (RA) was investigated in THP-1 monocytic cells cultured with RA and stimulated with lipopolysaccharide (LPS), tumour necrosis factor-alpha (TNF-alpha), interferon-beta (IFN-beta), and IFN-gamma, individually or in combinations. While RA (10(-8) m) alone did not alter STAT-1 activation or expression in THP-1 cells, RA enhanced or prolonged STAT-1 activation (tyrosine 701 phosphorylation) and gene expression (mRNA and protein) induced by either IFN-beta or IFN-gamma. However, in contrast, RA reduced STAT-1 activation and gene expression induced by LPS and/or TNF-alpha by about 50-70%, and lowered in vitro DNA binding activity to both a STAT-1 consensus element and a nuclear factor kappa B (NFkappaB) binding element. These results imply that RA can significantly rebalance STAT-1-dependent responses, and that one of the mechanisms may be through the inhibition of the NFkappaB pathway.

Interferons inhibit tumor necrosis factor-alpha-mediated matrix metalloproteinase-9 activation via interferon regulatory factor-1 binding competition with NF-kappa B

Enhanced expression of matrix metalloproteinase-9 (MMP-9) correlates with invasion during tumor progression. Interferons (IFNs) inhibit MMP-9 activation in response to tumor necrosis factor-alpha (TNF-alpha), and the latter activates the MMP-9 gene through NF-kappaB. Understanding the molecular basis for MMP-9 inhibition may provide tools to control cell invasion. The data reported here show the critical role of interferon regulatory factor-1 (IRF1) in the inhibition of MMP-9. (i) IFN treatment suppresses TNF-alpha-induced MMP-9 reporter activity in STAT1(+/+) cells but not in STAT1(-/-) cells. (ii) IRF1 transfection blocks TNF-alpha-mediated MMP-9 activation. (iii) IFNs phosphorylate STAT1 and induce IRF1 but do not affect Ikappa-B degradation nor NF-kappaB nuclear translocation. (iv) Nuclear NF-kappaB (p50/p65) and IRF1, but not STAT1, bind to the MMP-9 promoter region containing an IFN-responsive-like element overlapping the NF-kappaB-binding site. (v) Recombinant IRF1, although unable to bind to an NF-kappaB consensus sequence, competes with NF-kappaB proteins for binding to the MMP-9 promoter. (vi) Conversely recombinant p50/p65 proteins reduce IRF1-DNA binding. (vii) In cells cotransfected with IRF1 and/or p65 expression vectors, an excess of IRF1 reduces MMP-9 reporter activity, whereas an excess of p65 blocks the inhibitory effect of IFN-gamma. Thus, in contrast to the known synergism between IRF1 and NF-kappaB, our data identify a novel role for IRF1 as a competitive inhibitor of NF-kappaB binding to the particular MMP-9 promoter context.

STAT4 serine phosphorylation is critical for IL-12-induced IFN-gamma production but not for cell proliferation

T helper 1 (T(H)1) differentiation and IFN-gamma production are crucial in cell-mediated immune responses. IL-12 is an important regulator of this process and mediates its effects through signal transducer and activator of transcription 4 (STAT4). IFN-gamma production is also regulated by the p38 mitogen-activated kinase pathway, although the mechanisms are ill-defined. We show here that GADD45-beta and GADD45-gamma can induce STAT4 S721 phosphorylation via the MKK6/p38 pathway. Thus, STAT4 could be a target that accounts for the defects in cell-mediated immunity associated with perturbations in the p38 pathway. To investigate the biological significance of STAT4 S721 phosphorylation, we reconstituted primary spleen cells from STAT4-deficient mice with wild-type and mutated STAT4, by using a retroviral gene transduction. We demonstrated that expression of wild-type STAT4, but not the S721A mutant, restored normal T(H)1 differentiation and IFN-gamma synthesis. The inability of STAT4 S721 to restore IFN-gamma production was not caused by decreased IL-12R expression because the STAT4 S721 mutant also failed to restore IFN-gamma production in STAT4-deficient IL-12Rbeta2 transgenic cells. Importantly, STAT4 S721A-transduced cells showed normal proliferative response to IL-12, illustrating that serine phosphorylation is not required for IL-12-induced proliferation. Additionally, the results imply the existence of STAT4 serine phosphorylation-dependent and -independent target genes. We conclude that phosphorylation of STAT4 on both tyrosine and serine residues is important in promoting normal T(H)1 differentiation and IFN-gamma secretion.

Interleukin-22 (IL-22) activates the JAK/STAT, ERK, JNK, and p38 MAP kinase pathways in a rat hepatoma cell line. Pathways that are shared with and distinct from IL-10

IL (interleukin)-22 is an IL-10-related cytokine; its main biological activity known thus far is the induction of acute phase reactants in liver and pancreas. IL-22 signals through a receptor that is composed of two chains from the class II cytokine receptor family: IL-22R (also called ZcytoR11/CRF2-9) and IL-10Rbeta (CRF2-4), which is also involved in IL-10 signaling. In this report, we analyzed the signal transduction pathways activated in response to IL-22 in a rat hepatoma cell line, H4IIE. We found that IL-22 induces activation of JAK1 and Tyk2 but not JAK2, as well as phosphorylation of STAT1, STAT3, and STAT5 on tyrosine residues, extending the similarities between IL-22 and IL-10. However our results unraveled some differences between IL-22 and IL-10 signaling. Using antibodies specific for the phosphorylated form of MEK1/2, ERK1/2, p90RSK, JNK, and p38 kinase, we showed that IL-22 activates the three major MAPK pathways. IL-22 also induced serine phosphorylation of STAT3 on Ser(727). This effect, which is not shared with IL-10, was only marginally affected by MEK1/2 inhibitors, indicating that other pathways might be involved. Finally, by overexpressing a STAT3 S727A mutant, we showed that serine phosphorylation is required to achieve maximum transactivation of a STAT responsive promoter upon IL-22 stimulation.

The serine protease plasmin triggers expression of MCP-1 and CD40 in human primary monocytes via activation of p38 MAPK and janus kinase (JAK)/STAT signaling pathways

The mechanism of proinflammatory activation of human monocytes by plasmin is unknown. Here we demonstrate that in human primary monocytes, plasmin stimulates mitogen-activated protein kinase (MAPK) signaling via phosphorylation of MAPK kinase 3/6 (MKK3/6) and p38 MAPK that triggers subsequent DNA binding of transcription factor activator protein-1 (AP-1). The AP-1 complex contained phosphorylated c-Jun and ATF2, and its DNA binding activity was blocked by the p38 MAPK inhibitor SB203580. In addition, plasmin elicits Janus kinase (JAK)/signal transducer and activator of transcription (STAT) signaling, as detected by phosphorylation of JAK1 tyrosine kinase and STAT1 and STAT3 proteins. Plasmin-induced DNA binding of STAT1 and STAT3 was blocked by SB203580 and AG490, inhibitors of p38 MAPK and JAK, respectively, but not by U0126, an inhibitor of MKK1/2. DNA binding of NF-kappaB remained unaffected by any of these inhibitors. The plasmin-induced signaling led to expression of monocyte chemoattractant protein-1 (MCP-1) and CD40, which required activation of both p38 MAPK and JAK/STAT signaling pathways. Additionally, signaling through both p38 MAPK and JAK is involved in the plasmin-mediated monocyte migration, whereas the formylmethionylleucylphenylalanine-induced chemotaxis remained unaffected. Taken together, our data demonstrate a novel function of the serine protease plasmin in a proinflammatory signaling network.

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.

CPAP is a novel stat5-interacting cofactor that augments stat5-mediated transcriptional activity

Stat5, a member of the signal transducer and activators of transcription (Stat) protein family, is a primary mediator of prolactin (PRL) signaling in the mammary gland. There are two distinct Stat5 genes, Stat5a and Stat5b. The Stat5a isoform has been demonstrated to have an essential role in mammary epithelial differentiation, whereas Stat5b is required for dimorphic sexual growth. To search for proteins that interact with the C terminus of Stat5a, a highly divergent region amongst Stat family members, we performed a yeast two-hybrid screen of HBL100 and primary breast adenocarcinoma libraries. This led to the identification of a protein that had previously been isolated as a centrosomal P4.1-associated protein (CPAP). CPAP was shown to specifically interact with Stat5a and Stat5b but not with Stat1 or Stat3. Both the tyrosine phosphorylated and unphosphorylated forms of Stat5, as well as Stat5a/Stat5b heterodimers, could associate with CPAP. CPAP was expressed in human breast cancer cell lines and the developing mammary gland as well as in other tissues. Indirect immunofluorescence and cellular fractionation studies revealed that CPAP was predominantly cytoplasmic, with low levels in the nucleus. Nuclear levels of CPAP increased substantially upon activation of the PRL pathway, most likely reflecting cotranslocation of this protein with activated Stat5. Furthermore, CPAP was found to augment Stat5-mediated transcription. Thus, we have identified CPAP as a novel coactivator of Stat5 proteins in the PRL (and probably other) pathways.

Sensitization of IFN-gamma Jak-STAT signaling during macrophage activation

A general paradigm in signal transduction is ligand-induced feedback inhibition and the desensitization of signaling. We found that subthreshold concentrations of interferon-gamma (IFN-gamma), which did not activate macrophages, increased their sensitivity to subsequent IFN-gamma stimulation; this resulted in increased signal transducer and activator of transcription 1 (STAT1) activation and increased IFN-gamma#150;dependent gene activation. Sensitization of IFN-gamma signaling was mediated by the induction of STAT1 expression by low doses of IFN-gamma that did not effectively induce feedback inhibition. IFN-gamma signaling was sensitized in vivo after IFN-gamma injection, and STAT1 expression was increased after injection of lipopolysaccharide and in rheumatoid arthritis synovial cells. These results identify a mechanism that sensitizes macrophages to low concentrations of IFN-gamma and regulates IFN-gamma responses in acute and chronic inflammation.

Functional inactivation of CXC chemokine receptor 4-mediated responses through SOCS3 up-regulation

Hematopoietic cell growth, differentiation, and chemotactic responses require coordinated action between cytokines and chemokines. Cytokines promote receptor oligomerization, followed by Janus kinase (JAK) kinase activation, signal transducers and transactivators of transcription (STAT) nuclear translocation, and transcription of cytokine-responsive genes. These include genes that encode a family of negative regulators of cytokine signaling, the suppressors of cytokine signaling (SOCS) proteins. After binding their specific receptors, chemokines trigger receptor dimerization and activate the JAK/STAT pathway. We show that SOCS3 overexpression or up-regulation, stimulated by a cytokine such as growth hormone, impairs the response to CXCL12, measured by Ca(2+) flux and chemotaxis in vitro and in vivo. This effect is mediated by SOCS3 binding to the CXC chemokine receptor 4 receptor, blocking JAK/STAT and Galpha(i) pathways, without interfering with cell surface chemokine receptor expression. The data provide clear evidence for signaling cross-talk between cytokine and chemokine responses in building a functional immune system.

Leptin signalling

The identification of leptin as the product of the obesity (ob) gene has been followed by extensive research identifying a wide spectrum of physiological effects elicited by this adipose-derived hormone. These effects are mediated via a family of cytokine-like receptor isoforms distributed in both the central nervous system and periphery. The signal transduction pathways regulated by leptin are diverse and include those characteristic of both cytokine and growth factor receptor signalling. This review describes the structure and function of leptin receptors and summarizes recent progress that has been made in characterizing the increasing number of signal transduction pathways regulated by leptin.

Review: IFN-alpha/beta receptor interactions to biologic outcomes: understanding the circuitry

Type I interferons (IFNs), which include the IFN-alphas, IFN-beta, IFN-omega, IFN-kappa, and IFN-tau, are an evolutionarily conserved group of secreted cytokines that serve as potent extracellular mediators of host defense and homeostasis. Binding of IFNs to specific cell surface receptors results in the activation of multiple intracellular signaling cascades, leadingto the synthesis of proteins that mediate antiviral, growth inhibitory and immunomodulatory responses. In the past decade, considerable information has accumulated pertaining to the different signalingpathways that are activated by the type I IFNs. Although many of the literature findings are specific to defined cell systems or are tissue restricted, the intent of this review is to place these signaling cascades and their effectors in the context of distinct biologic outcomes.

Low expression of interferon-stimulated genes in active multiple sclerosis is linked to subnormal phosphorylation of STAT1

Multiple sclerosis is an immune-mediated brain disease ameliorated by interferon-beta therapy. Immune responses to IFN-alpha and IFN-beta are sometimes subnormal in MS peripheral blood mononuclear cells (MNCs), suggesting an underlying defect in type I IFN signaling. We studied IFN-beta regulation of mRNA and protein induction for IFN regulatory factor-1 (IRF-1) and IRF-2, which control multiple IFN-stimulated genes, and for 2',5'-oligoadenylate synthetase (2',5'-OAS) and MxA, which are antiviral proteins. First, mRNA levels in resting MNC from untreated patients with clinically active MS contained IRF-1 at 38% of normal controls, 45% for IRF-2, 44% for 2',5'-OAS (all p<0.005), and 46% for MxA protein (p<0.007). Stable MS patients had intermediate levels of 2',5'-OAS and MxA. IFN-beta-1b therapy increased IRF-1, IRF-2, and 2',5'-OAS mRNA in resting MNC-but only up to levels seen in unstimulated control cells. In untreated patients with active MS, serine phosphorylation of the STAT1 transcription factor was markedly reduced, suggesting a mechanism for the low levels of IFN-induced genes. Secondly, in untreated patients with stable MS, culture with IFN-beta induced excessive tyrosine phosphorylation of STAT1, and this correlated with low SHP1 tyrosine phosphatase levels. Excessive P-Tyr-STAT1 responses could induce inflammatory cytokines and demyelination in MS, as in motheaten mice, which have defects in SHP-1 function. Abnormal IFN signaling may predict the course of MS and responses to therapy.

gamma-Glutamyl transpeptidase expression in Ewing's sarcoma cells: up-regulation by interferons

The genetic hallmark of Ewing's sarcoma family of tumours (ET) is the presence of the translocation t(11;22)(q24;q12), which creates the ET fusion gene, leading to cellular transformation. Five human gamma-glutamyl transpeptidase (gamma-GT) genes are located near the chromosomal translocation in ET. gamma-GT is a major enzyme involved in glutathione homoeostasis. Five human cell lines representative of primary or metastatic tumours were investigated to study whether gamma-GT alterations could occur at the chromosomal breaks and rearrangements in ET. As shown by enzymic assays and FACS analyses, all ET cell lines consistently expressed a functional gamma-GT which however did not discriminate steps of ET progression. As shown previously [Sancéau, Hiscott, Delattre and Wietzerbin (2000) Oncogene 19, 3372-3383], ET cells respond to the antiproliferative effects of interferons (IFNs) type I (alpha and beta) and to a much less degree to IFN type II (gamma). IFN-alpha and -beta arrested cells in the S-phase of the cell cycle. We found an enhancement of gamma-GT mRNA species with IFN-alpha and -beta by reverse transcriptase-PCR analyses. This is reflected by up-regulation of gamma-GT protein, which coincides with the increase in gamma-GT-specific enzymic activity. Similarly, IFNs up-regulate the levels of gamma-GT in another IFN-responsive B cell line. Whether this up-regulation of gamma-GT by IFNs is of physiological relevance to cell behaviour remains to be studied.

Interferon-beta activates multiple signaling cascades in primary human microglia

Microglia, the resident brain macrophages, are the principal cells involved in the regulation of inflammatory and antimicrobial responses in the CNS. Interferon-beta (IFNbeta) is an antiviral cytokine induced by viral infection or following non-specific inflammatory challenges of the CNS. Because of the well-known anti-inflammatory properties of IFNbeta, it is also used to treat multiple sclerosis, an inflammatory CNS disease. Despite the importance of IFNbeta signaling in CNS cells, little has been studied, particularly in microglia. In this report, we investigated the molecular mechanisms underlying IFNbeta-induced beta-chemokine expression in primary human fetal microglia. Multiple signaling cascades are activated in microglia by IFNbeta, including nuclear factor-kappaB (NF-kappaB), activator protein-1 (AP-1) and Jak/Stat. IFNbeta induced IkappaBalpha degradation and NF-kappaB (p65:p50) DNA binding. Inhibition of NF-kappaB by either adenoviral transduction of a super repressor IkappaBalpha, or an antioxidant inhibitor of NF-kappaB reduced expression of the beta-chemokines, regulated upon activation, normal T-cell expressed and secreted (RANTES) and macrophage inflammatory protein (MIP)-1beta. IFNbeta also induced phosphorylation of extracellular signal-regulated kinase (ERK) mitogen-activated protein kinase, and the MAP kinase kinase 1 (MEK1) inhibitor PD98059 dose-dependently inhibited beta-chemokine mRNA and protein expression. PD98059 did not inhibit NF-kappaB binding, demonstrating that ERK was not responsible for NF-kappaB activation. Two downstream targets of ERK were identified in microglia: AP-1 and Stat1. IFNbeta induced AP-1 nuclear binding activity in microglia and this was suppressed by PD98059. Additionally, IFNbeta induced Stat1 phosphorylation at both tyrosine 701 (Y701) and serine 727 (S727) residues. S727 phosphorylation of Stat1, which is known to be required for maximal transcriptional activation, was inhibited by PD98059. Our results demonstrating multiple signaling cascades initiated by IFNbeta in primary human microglia are novel and have implications for inflammatory and infectious diseases of the CNS.

Role of the JAK-STAT pathway in PDGF-stimulated proliferation of human airway smooth muscle cells

Airway remodeling, as manifested by an increase in airway smooth muscle mass, mucous gland hyperplasia, and subepithelial fibrosis, contributes to the airway hyperresponsiveness and fixed obstruction seen in some asthmatic patients. Here we investigated whether the Janus kinase-signal transducer and activator of transcription (JAK-STAT) pathway contributes to platelet-derived growth factor (PDGF)-stimulated mitogenesis of human airway smooth muscle cells (HASMC). PDGF treatment of quiescent HASMC resulted in the rapid tyrosine phosphorylation and DNA binding of STAT1 and STAT3. This phosphorylation was blocked by inhibition of Src and JAK2 kinases. In addition, STAT activation by PDGF was found to be redox dependent. Moreover, PDGF-induced thymidine uptake was completely blocked by pretreatment of HASMC with the STAT kinase inhibitors AG-490, SU-6656, and PP2. Interestingly, the JAK pathway was required for HASMC mitogenesis independently of mitogen-activated protein kinase activation. Inhibition of the Src and JAK kinases blocked PDGF-stimulated gene expression of the STAT target genes cyclin D1 and c-myc. These results indicate that the JAK-STAT pathway contributes to PDGF-induced mitogenesis, and thus this pathway may be important in the airway remodeling seen in some asthmatic patients.

The synergy between stem cell factor (SCF) and granulocyte colony-stimulating factor (G-CSF): molecular basis and clinical relevance

Stem cell factor (SCF), an essential growth factor in normal hematopoiesis, exerts potent effects when combined with cytokines. In particular, its synergy with granulocyte colony-stimulating factor (G-CSF) results in important biologic responses. These include enhancement of ex vivo long-term expansion of human primitive hematopoietic cells and increased mobilization of peripheral blood progenitor cells (PBPC) for transplantation. Despite the clinical importance of the interaction between SCF and G-CSF, the absence of a model system in which it could be studied at the cellular level had impaired the ability to understand the basis of their co-operation. To overcome this impediment, a system was recently generated which recapitulates the biologic synergy between SCF and G-CSF. MO7e-G cells have allowed the identification of key events in the synergistic actions of these cytokines on proliferation and gene expression. Among the biochemical and molecular events mediated by these cytokines are the down-regulation of p27kip1 and the independent phosphorylation of STAT3 on tyrosine and serine residues. Recent work has provided increasing evidence for the clinical importance of the combination of SCF and G-CSF. The elucidation of the intracellular events triggered by their receptors is now shedding light on key mediators of their synergistic effects. The identification of these pathways is of considerable importance for understanding fundamental aspects of hematopoiesis, and as potential targets for therapeutic intervention.

A road map for those who don't know JAK-STAT

The Janus kinase-signal transducer and activator of transcription (JAK-STAT) pathway transmits information received from extracellular polypeptide signals, through transmembrane receptors, directly to target gene promoters in the nucleus, providing a mechanism for transcriptional regulation without second messengers. Evolutionarily conserved in eukaryotic organisms from slime molds to humans, JAK-STAT signaling appears to be an early adaptation to facilitate intercellular communication that has co-evolved with myriad cellular signaling events. This co-evolution has given rise to highly adapted, ligand-specific signaling pathways that control gene expression. In addition, the JAK-STAT signaling pathways are regulated by a vast array of intrinsic and environmental stimuli, which can add plasticity to the response of a cell or tissue.