Stat3 oxidation-dependent regulation of gene expression impacts on developmental processes and involves cooperation with Hif-1α

Reactive oxygen species are bona fide intracellular second messengers that influence cell metabolism and aging by mechanisms that are incompletely resolved. Mitochondria generate superoxide that is dis-mutated to hydrogen peroxide, which in turn oxidises cysteine-based enzymes such as phosphatases, peroxiredoxins and redox-sensitive transcription factors to modulate their activity. Signal Transducer and Activator of Transcription 3 (Stat3) has been shown to participate in an oxidative relay with peroxiredoxin II but the impact of Stat3 oxidation on target gene expression and its biological consequences remain to be established. Thus, we created murine embryonic fibroblasts (MEFs) that express either WT-Stat3 or a redox-insensitive mutant of Stat3 (Stat3-C3S). The Stat3-C3S cells differed from WT-Stat3 cells in morphology, proliferation and resistance to oxidative stress; in response to cytokine stimulation, they displayed elevated Stat3 tyrosine phosphorylation and Socs3 expression, implying that Stat3-C3S is insensitive to oxidative inhibition. Comparative analysis of global gene expression in WT-Stat3 and Stat3-C3S cells revealed differential expression (DE) of genes both under basal conditions and during oxidative stress. Using differential gene regulation pattern analysis, we identified 199 genes clustered into 10 distinct patterns that were selectively responsive to Stat3 oxidation. GO term analysis identified down-regulated genes to be enriched for tissue/organ development and morphogenesis and up-regulated genes to be enriched for cell-cell adhesion, immune responses and transport related processes. Although most DE gene promoters contain consensus Stat3 inducible elements (SIEs), our chromatin immunoprecipitation (ChIP) and ChIP-seq analyses did not detect Stat3 binding at these sites in control or oxidant-stimulated cells, suggesting that oxidised Stat3 regulates these genes indirectly. Our further computational analysis revealed enrichment of hypoxia response elements (HREs) within DE gene promoters, implying a role for Hif-1. Experimental validation revealed that efficient stabilisation of Hif-1α in response to oxidative stress or hypoxia required an oxidation-competent Stat3 and that depletion of Hif-1α suppressed the inducible expression of Kcnb1, a representative DE gene. Our data suggest that Stat3 and Hif-1α cooperate to regulate genes involved in immune functions and developmental processes in response to oxidative stress.

STAT3 suppresses Wnt/β-catenin signaling during the induction phase of primary Myf5+ brown adipogenesis

Thermogenic fat is a promising target for new therapies in diabetes and obesity. Understanding how thermogenic fat develops is important to develop rational strategies to treat obesity. Previously, we have shown that Tyk2 and STAT3, part of the JAK-STAT pathway, are necessary for proper development of classical brown fat. Using primary preadipocytes isolated from newborn mice we demonstrate that STAT3 is required for differentiation and robust expression of Uncoupling Protein 1 (UCP1). We also confirm that STAT3 is necessary during the early induction stage of differentiation and is dispensable during the later terminal differentiation stage. The inability of STAT3^-/- preadipocytes to differentiate can be rescued using Wnt ligand secretion inhibitors when applied during the induction stage. Through chemical inhibition and RNAi, we show that it is the canonical β-catenin pathway that is responsible for the block in differentiation; inhibition or knockdown of β-catenin can fully rescue adipogenesis and UCP1 expression in the STAT3^-/- adipocytes. During the induction stage, Wnts 1, 3a, and 10b have increased expression in the STAT3^-/- adipocytes, potentially explaining the increased levels and activity of β-catenin. Our results for the first time point towards an interaction between the JAK/STAT pathway and the Wnt/β-catenin pathway during the early stages of in-vitro adipogenesis.

Stress-induced dynamic regulation of mitochondrial STAT3 and its association with cyclophilin D reduce mitochondrial ROS production

Signal transducer and activator of transcription 3 (STAT3) is associated with various physiological and pathological functions, mainly as a transcription factor that translocates to the nucleus upon tyrosine phosphorylation induced by cytokine stimulation. In addition, a small pool of STAT3 resides in the mitochondria, where it serves as a sensor for various metabolic stressors including reactive oxygen species (ROS). Mitochondrially localized STAT3 largely exerts its effects through direct or indirect regulation of the activity of the electron transport chain (ETC). It has been assumed that the amounts of STAT3 in the mitochondria are static. We showed that various stimuli, including oxidative stress and cytokines, triggered a signaling cascade that resulted in a rapid loss of mitochondrially localized STAT3. Recovery of the mitochondrial pool of STAT3 over time depended on phosphorylation of Ser727 in STAT3 and new protein synthesis. Under these conditions, mitochondrially localized STAT3 also became competent to bind to cyclophilin D (CypD). Binding of STAT3 to CypD was mediated by the amino terminus of STAT3, which was also important for reducing mitochondrial ROS production after oxidative stress. These results outline a role for mitochondrially localized STAT3 in sensing and responding to external stimuli.

Kinase Inactive Tyrosine Kinase (Tyk2) Supports Differentiation of Brown Fat Cells

It has been known for decades that brown adipose tissue (BAT) plays a central role in maintaining body temperature in hibernating animals and human infants. Recently, it has become evident that there are also depots of brown fat in adult humans, and the mass of brown fat is inversely correlated with body weight. There are a variety of transcription factors implicated in the differentiation of classical Myf5+ brown preadipocytes, one of the most important of which is PRDM16. We have recently identified that in addition to PRDM16, the tyrosine kinase Tyk2 and the STAT3 transcription factor are required for the differentiation of Myf5 positive brown preadipocytes both in cell culture and in mice. Tyk2 is a member of the Jak family of tyrosine kinases, which are activated by exposure of cells to different cytokines and growth factors. In this study we report the surprising observation that a mutated form of Tyk2, which lacks tyrosine kinase activity (Tyk2KD) restores differentiation of brown preadipocytes in vitro as well as in Tyk2-/- mice. Furthermore, expression of the Tyk2KD transgene in brown fat reverses the obese phenotype of Tyk2-/- animals. Treatment of cells with Jak-selective inhibitors suggests that the mechanism by which Tyk2KD functions to restore BAT differentiation is by dimerizing with kinase active Jak1 or Jak2. These results indicate that there are redundant mechanisms by which members of the Jak family can contribute to differentiation of BAT.

The Signal Transducer and Activator of Transcription 1 (STAT1) Inhibits Mitochondrial Biogenesis in Liver and Fatty Acid Oxidation in Adipocytes

The transcription factor STAT1 plays a central role in orchestrating responses to various pathogens by activating the transcription of nuclear-encoded genes that mediate the antiviral, the antigrowth, and immune surveillance effects of interferons and other cytokines. In addition to regulating gene expression, we report that STAT1^-/- mice display increased energy expenditure and paradoxically decreased release of triglycerides from white adipose tissue (WAT). Liver mitochondria from STAT1^-/- mice show both defects in coupling of the electron transport chain (ETC) and increased numbers of mitochondria. Consistent with elevated numbers of mitochondria, STAT1^-/- mice expressed increased amounts of PGC1α, a master regulator of mitochondrial biogenesis. STAT1 binds to the PGC1α promoter in fed mice but not in fasted animals, suggesting that STAT1 inhibited transcription of PGC1α. Since STAT1^-/- mice utilized more lipids we examined white adipose tissue (WAT) stores. Contrary to expectations, fasted STAT1^-/- mice did not lose lipid from WAT. β-adrenergic stimulation of glycerol release from isolated STAT1^-/- WAT was decreased, while activation of hormone sensitive lipase was not changed. These findings suggest that STAT1^-/- adipose tissue does not release glycerol and that free fatty acids (FFA) re-esterify back to triglycerides, thus maintaining fat mass in fasted STAT1^-/- mice.

Toward a new STATe: the role of STATs in mitochondrial function

Signal Transducers and Activators of Transcription (STATs) have been studied extensively and have been associated with virtually every biochemical pathway. Until recently, however, they were thought to exert these effects solely as a nuclear transcription factor. The finding that STAT3 localizes to the mitochondria and modulates respiration has opened up a new avenue through which STATs may regulate the cell. Recently, other members of the STAT family (STAT1, STAT2, STAT5, and STAT6) have also been shown to be present in the mitochondria. Coordinate regulation at the nucleus and mitochondria by these proteins places them in a unique position to drive cellular processes to achieve a specific response. This review summarizes recent findings that have led to our current understanding of how STATs influence mitochondrial function in health and disease.

Mitochondrial localized Stat3 promotes breast cancer growth via phosphorylation of serine 727

Signal transducer and activator of transcription 3 (Stat3) is a key mediator in the development of many cancers. For 20 years, it has been assumed that Stat3 mediates its biological activities as a nuclear localized transcription factor activated by many cytokines. However, recent studies from this laboratory and others indicate that Stat3 has an independent function in the mitochondria (mitoStat3) where it controls the activity of the electron transport chain (ETC) and mediates Ras-induced transformation of mouse embryo fibroblasts. The actions of mitoStat3 in controlling respiration and Ras transformation are mediated by the phosphorylation state of serine 727. To address the role of mitoStat3 in the pathogenesis of cells that are transformed, we used 4T1 breast cancer cells, which form tumors that metastasize in immunocompetent mice. Substitution of Ser-727 for an alanine or aspartate in Stat3 that has a mitochondrial localization sequence, MLS-Stat3, has profound effects on tumor growth, complex I activity of the ETC, and accumulation of reactive oxygen species (ROS). Cells expressing MLS-Stat3(S727A) display slower tumor growth, decreased complex I activity of the ETC, and increased ROS accumulation under hypoxia compared with cells expressing MLS-Stat3. In contrast, cells expressing MLS-Stat3(S727D) show enhanced tumor growth and complex I activity and decreased production of ROS. These results highlight the importance of serine 727 of mitoStat3 in breast cancer and suggest a novel role for mitoStat3 in regulation of ROS concentrations through its action on the ETC.

The import of the transcription factor STAT3 into mitochondria depends on GRIM-19, a component of the electron transport chain

The signal transducer and activator of transcription 3 (STAT3), a nuclear transcription factor, is also present in mitochondria and regulates cellular respiration in a transcriptional-independent manner. The mechanism of STAT3 import into mitochondria remains obscure. In this report we show that mitochondrial-localized STAT3 resides in the inner mitochondrial membrane. In vitro import studies show that the gene associated with retinoid interferon induced cell mortality 19 (GRIM-19), a complex I subunit that acts as a chaperone to recruit STAT3 into mitochondria. In addition, GRIM-19 enhances the integration of STAT3 into complex I. A S727A mutation in STAT3 reduces its import and assembly even in the presence of GRIM-19. Together, our studies unveil a novel chaperone function for GRIM-19 in the recruitment of STAT3 into mitochondria.

Tyk2 and Stat3 regulate brown adipose tissue differentiation and obesity

Mice lacking the Jak tyrosine kinase member Tyk2 become progressively obese due to aberrant development of Myf5+ brown adipose tissue (BAT). Tyk2 RNA levels in BAT and skeletal muscle, which shares a common progenitor with BAT, are dramatically decreased in mice placed on a high-fat diet and in obese humans. Expression of Tyk2 or the constitutively active form of the transcription factor Stat3 (CAStat3) restores differentiation in Tyk2(-/-) brown preadipocytes. Furthermore, Tyk2(-/-) mice expressing CAStat3 transgene in BAT also show improved BAT development, normal levels of insulin, and significantly lower body weights. Stat3 binds to PRDM16, a master regulator of BAT differentiation, and enhances the stability of PRDM16 protein. These results define Tyk2 and Stat3 as critical determinants of brown fat lineage and suggest that altered levels of Tyk2 are associated with obesity in both rodents and humans.

Multi-tasking: nuclear transcription factors with novel roles in the mitochondria

Coordinated responses between the nucleus and mitochondria are essential for the maintenance of homeostasis. For over 15 years, pools of nuclear transcription factors (TFs), such as p53 and nuclear hormone receptors, have been observed in the mitochondria. The contribution of the mitochondrial pool of these TFs to their well-defined biological actions is in some cases clear and in others not well understood. Recently, a small mitochondrial pool of the TF signal transducer and activator of transcription factor 3 (STAT3) was shown to modulate the activity of the electron transport chain (ETC). The mitochondrial function of STAT3 encompasses both its biological actions in the heart as well as its oncogenic effects. This review highlights advances in our understanding of how mitochondrial pools of nuclear TFs may influence the function of this organelle.

Cytoprotection by the modulation of mitochondrial electron transport chain: the emerging role of mitochondrial STAT3

The down regulation of mitochondrial electron transport is an emerging mechanism of cytoprotective intervention that is effective in pathologic settings such as myocardial ischemia and reperfusion when the continuation of mitochondrial respiration produces reactive oxygen species, mitochondrial calcium overload, and the release of cytochrome c to activate cell death programs. The initial target of deranged electron transport is the mitochondria themselves. In the first part of this review, we describe this concept and summarize different approaches used to regulate mitochondrial respiration by targeting complex I as a proximal site in the electron transport chain (ETC) in order to favor the cytoprotection. The second part of the review highlights the emerging role of signal transducer and activator of transcription 3 (STAT3) in the direct, non-transcriptional regulation of ETC, as an example of a genetic approach to modulate respiration. Recent studies indicate that a pool of STAT3 resides in the mitochondria where it is necessary for the maximal activity of complexes I and II of the electron transport chain (ETC). The overexpression of mitochondrial-targeted STAT3 results in a partial blockade of electron transport at complexes I and II that does not impair mitochondrial membrane potential nor enhance the production of reactive oxygen species (ROS). The targeting of transcriptionally-inactive STAT3 to mitochondria attenuates damage to mitochondria during cell stress, resulting in decreased production of ROS and retention of cytochrome c by mitochondria. The overexpression of STAT3 targeted to mitochondria unveils a novel protective approach mediated by modulation of mitochondrial respiration that is independent of STAT3 transcriptional activity. The limitation of mitochondrial respiration under pathologic circumstances can be approached by activation and overexpression of endogenous signaling mechanisms in addition to pharmacologic means. The regulation of mitochondrial respiration comprises a cardioprotective paradigm to decrease cellular injury during ischemia and reperfusion.

The role of Tyk2 in regulation of breast cancer growth

The antigrowth and immunomodulatory actions of interferons (IFNs) have enabled these cytokines to be used therapeutically for the treatment of a variety of hematologic and solid malignancies. IFNs exert their effects by activation of the Jak/Stat signaling pathway. IFNγ stimulates the tyrosine kinases Jak1 and Jak2, resulting in activation of the Stat1 transcription factor, whereas type 1 IFNs (IFNα/β) activate Jak1 and Tyk2, which mediate their effects through Stat1 and Stat2. Disruption in the expression of IFNγ, IFNα receptors, or Stat1 inhibits antitumor responses and blunt cancer immunosurveillance in mice. Mutations in Jak2 or constitutive activation of Jak1 or Jak2 also promote the development of a variety of malignancies. Although there are data indicating that Tyk2 plays a role in the pathogenesis of lymphomas, the effects of Tyk2 expression on tumorigenesis are unknown. We report here that Tyk2(-/-) mice inoculated with 4T1 breast cancer cells show enhanced tumor growth and metastasis compared to Tyk2(+/+) animals. Accelerated growth of 4T1 cells in Tyk2(-/-) animals does not appear to be due to decreased function of CD4(+), CD8(+) T cells, or NK cells. Rather, the tumor suppresive effects of Tyk2 are mediated at least in part by myeloid-derived suppressor cells, which appear to be more effective in inhibiting T cell responses in Tyk2(-/-) mice. Our results provide the first evidence for a role of Tyk2 in suppressing the growth and metastasis of breast cancer.

Mitochondrial-targeted Signal transducer and activator of transcription 3 (STAT3) protects against ischemia-induced changes in the electron transport chain and the generation of reactive oxygen species

Expression of the STAT3 transcription factor in the heart is cardioprotective and decreases the levels of reactive oxygen species. Recent studies indicate that a pool of STAT3 resides in the mitochondria where it is necessary for the maximal activity of complexes I and II of the electron transport chain. However, it has not been explored whether mitochondrial STAT3 modulates cardiac function under conditions of stress. Transgenic mice with cardiomyocyte-specific overexpression of mitochondria-targeted STAT3 with a mutation in the DNA-binding domain (MLS-STAT3E) were generated. We evaluated the role of mitochondrial STAT3 in the preservation of mitochondrial function during ischemia. Under conditions of ischemia heart mitochondria expressing MLS-STAT3E exhibited modest decreases in basal activities of complexes I and II of the electron transport chain. In contrast to WT hearts, complex I-dependent respiratory rates were protected against ischemic damage in MLS-STAT3E hearts. MLS-STAT3E prevented the release of cytochrome c into the cytosol during ischemia. In contrast to WT mitochondria, ischemia did not augment reactive oxygen species production in MLS-STAT3E mitochondria likely due to an MLS-STAT3E-mediated partial blockade of electron transport through complex I. Given the caveat of STAT3 overexpression, these results suggest a novel protective mechanism mediated by mitochondrial STAT3 that is independent of its canonical activity as a nuclear transcription factor.

Mitochondrial STAT3 supports Ras-dependent oncogenic transformation

Signal transducer and activator of transcription 3 (STAT3) is a latent cytoplasmic transcription factor responsive to cytokine signaling and tyrosine kinase oncoproteins by nuclear translocation when it is tyrosine-phosphorylated. We report that malignant transformation by activated Ras is impaired without STAT3, in spite of the inability of Ras to drive STAT3 tyrosine phosphorylation or nuclear translocation. Moreover, STAT3 mutants that cannot be tyrosine-phosphorylated, that are retained in the cytoplasm, or that cannot bind DNA nonetheless supported Ras-mediated transformation. Unexpectedly, STAT3 was detected within mitochondria, and exclusive targeting of STAT3 to mitochondria without nuclear accumulation facilitated Ras transformation. Mitochondrial STAT3 sustained altered glycolytic and oxidative phosphorylation activities characteristic of cancer cells. Thus, in addition to its nuclear transcriptional role, STAT3 regulates a metabolic function in mitochondria, supporting Ras-dependent malignant transformation.

Function of mitochondrial Stat3 in cellular respiration

Cytokines such as interleukin-6 induce tyrosine and serine phosphorylation of Stat3 that results in activation of Stat3-responsive genes. We provide evidence that Stat3 is present in the mitochondria of cultured cells and primary tissues, including the liver and heart. In Stat3(-/-) cells, the activities of complexes I and II of the electron transport chain (ETC) were significantly decreased. We identified Stat3 mutants that selectively restored the protein's function as a transcription factor or its functions within the ETC. In mice that do not express Stat3 in the heart, there were also selective defects in the activities of complexes I and II of the ETC. These data indicate that Stat3 is required for optimal function of the ETC, which may allow it to orchestrate responses to cellular homeostasis.

Tyk2 tyrosine kinase expression is required for the maintenance of mitochondrial respiration in primary pro-B lymphocytes

Tyk2, a member of the Jak family of protein tyrosine kinases, is critical for the biological actions of alpha/beta interferon (IFN-alpha/beta). Although Tyk2(-/-) mice are phenotypically normal, they exhibit abnormal responses to inflammatory challenges in a variety of cells isolated from Tyk2(-/-) mice. The reported phenotypic alterations in both Tyk2-null cells and mice are consistent with the possibility that the expression of this tyrosine kinase may regulate mitochondrial function. We report here that Tyk2-null pro-B cells are markedly deficient in basal oxygen consumption and exhibit a significant decrease in steady-state cellular ATP levels compared to wild-type cells. Tyk2-null cells also exhibit impaired complex I, III, and IV function of the mitochondrial electron transport chain. Reconstitution of Tyk2-null pro-B cells with either the wild type or a kinase-inactive mutant of Tyk2 restores basal mitochondrial respiration. By contrast, the kinase activity of Tyk2 is required for maintenance of both complex I-dependent mitochondrial respiration as well as induction of apoptosis in cells incubated with IFN-beta. Consistent with the role of Tyk2 in the regulation of tyrosine phosphorylation of Stat3, expression of a constitutively active Stat3 can restore the mitochondrial respiration in Tyk2-null cells treated with IFN-beta. Finally, Tyk2(-/-) mice show decreased exercise tolerance compared to wild-type littermates. Our results implicate a novel role for Tyk2 kinase and Stat3 phosphorylation in mitochondrial respiration.

Activation of Tyk2 and Stat3 is required for the apoptotic actions of interferon-beta in primary pro-B cells

The growth-inhibitory effects of type 1 interferons (IFNs) (IFNalpha/beta) are complex, and the role of apoptosis in their antigrowth effects is variable and not well understood. We have examined primary murine interleukin-7-dependent bone marrow-derived pro-B cells, where IFNbeta, but not IFNalpha, induces programmed cell death (PCD). IFNbeta-stimulated apoptosis is the same in pro-B cells derived from wild type and Stat1(-/-) mice. However, in pro-B cells from Tyk2(-/-) mice, where there is normal activation of Stat1 and Stat2, IFNbeta-stimulated PCD is not observed. Loss of B cells in lymphocytic choriomeningitis virus-infected mice has been shown to be mediated through the expression of IFNalpha/beta (1). In wild type mice infected with lymphocytic choriomeningitis virus, there is a greater loss of B cells in the bone marrow and spleen than in Tyk2(-/-) mice infected with the virus, suggesting that the expression of this kinase plays an in vivo role in IFNalpha/beta-mediated PCD. In contrast to IFNbeta-stimulated tyrosine phosphorylation of Stat1 and Stat2, Stat3 tyrosine phosphorylation is defective in Tyk2(-/-) pro-B cells, suggesting that this Stat family member is required for apoptosis. In support of this hypothesis, inhibition of Stat3 activation in wild type B cells reverses the apoptotic effects of IFNbeta. Furthermore, expression of a constitutively active form of Stat3 in Tyk2(-/-) B cells partially restores IFNbeta-stimulated PCD. These results demonstrate an important role of Tyk2-mediated tyrosine phosphorylation of Stat3 in the ability of IFNbeta to stimulate apoptosis of primary pro-B cells.

Type I interferons activate apoptosis in a Jurkat cell variant by caspase-dependent and independent mechanisms

Although the antiviral actions of interferons (IFNs) are observed in most types of cells, the antiproliferative effects of IFNalpha/beta are variable as are the mechanisms of growth inhibition that may or may not be due to the induction of apoptosis. To understand more about the mechanisms that are responsible for IFNalpha/beta-stimulated apoptosis, we have characterized a new human Jurkat T cell variant named H123 where IFNalpha activates programmed cell death (PCD). No differences in IFNalpha-stimulated, Stat-dependent gene expression were detected between H123 cells and the parental Jurkat cells, which are growth inhibited, but do not undergo apoptosis with IFNalpha. Although IFNalpha stimulates the activity of both caspase 3 and 9 in H123 cells, the general caspase inhibitor Z-VAD only partially reverses the apoptotic actions of IFNalpha. Induction of apoptosis by IFNalpha occurs through a mitochondrial-dependent pathway in H123 cells, as demonstrated by the release of cytochrome C from the mitochondria. Furthermore, IFNalpha treatment of H123 cells stimulates the release of the serine protease HtrA2/Omi from the mitochondria, suggesting that it plays a role in the apoptotic actions of this cytokine. These results provide evidence for a novel type 1 IFN-mediated pathway that regulates apoptosis of T cells through a mitochondrial-dependent and caspase-dependent and independent pathway.

Histone Deacetylase Activity Is Required to Recruit RNA Polymerase II to the Promoters of Selected Interferon-stimulated Early Response Genes

Posttranslational modification of histones by acetylation, methylation or phosphorylation has emerged as a major mechanism to modify chromatin structure and gene expression. In most cases, transcriptionally active genes display enhanced binding of acetylated histones in their promoters. Activation of histone acetyltransferases or inhibition of histone deacetylases (HDACs) allows chromatin to assume a more open state permitting transcriptional activators to form a preinitiation complex. To our surprise, treatment of cells with the HDAC inhibitor, trichostatin A (TSA), inhibits selected interferon beta (IFNbeta)-stimulated immediate early genes that are activated by the transcription factors Stat1 and Stat2. However, IFNbeta activation of IRF-1, which requires tyrosine-phosphorylated Stat1 homodimers binding to a gamma interferon activation sequence in its promoter is not affected by TSA. Exposure of cells to TSA does not alter tyrosine phosphorylation of Stat1 or Stat2. TSA treatment of cells also does not alter the binding of Stat 1 or Stat2 to the endogenous ISG54 promoter. However, IFNbeta-stimulated binding of RNA polymerase II to the ISG54 promoter is prevented by TSA. Interestingly, ectopic expression of IRF9 reverses the inhibitory actions of TSA, suggesting that IRF9 functions to recruit RNA polymerase II to the promoter of interferon-stimulated genes. This particular function of IRF9 requires the activity of histone deacetylases.

Identification of a novel conserved motif in the STAT family that is required for tyrosine phosphorylation

The rapid transcriptional activation of cellular genes by either type 1 interferons (IFNalpha/beta) or type 2 interferon (IFNgamma) is responsible for many of the pleiotropic effects of these cytokines, including their antiviral, antigrowth, and immunomodulatory activities. Interferon-stimulated gene expression is mediated by transcription factors termed Stats, which upon being tyrosine-phosphorylated, translocate to the nucleus and bind enhancers of interferon-activated genes. We have recently characterized a new Jurkat cell variant, named H123, where IFNalpha stimulates programmed cell death. H123 clones that are resistant to the apoptotic actions of IFNalpha have been selected. One of these clones (Clone 8) is defective in its responses to IFNalpha with regard to activation of genes that require tyrosine phosphorylation of Stat2. Stimulation of Clone 8 cells with IFNalpha induces normal tyrosine phosphorylation of Stat1 and Stat3. Sequencing of Stat2 RNA reveals a substitution of proline 630 located within the Src homology 2 domain of Stat2 to leucine (P630L). Pro-630 and its adjacent amino acids are conserved in all Stat family members but are absent in other proteins that contain Src homology 2 domains. Expression of Stat2 P630L in cells inhibits IFNalpha-stimulated gene expression. These results not only define a critical motif in Stat2 required for its transcriptional activity, but they also provide evidence that resistance to type one IFNs can be mediated by mutations in Stat2 as well as those previously described for Stat1.

Thrombomodulin RNA Is Destabilized Through Its 3′-Untranslated Element in Cells Exposed to IFN-γ

Interferon-gamma (IFN-gamma) is a potent activator of mononuclear phagocytes, allowing them to play a prominent role in acute and chronic inflammatory responses. IFN-gamma binding to its cell surface receptor initiates changes in the steady-state levels of cellular RNAs, permitting the proteins encoded by these RNAs to exert its biologic actions. Hundreds of cellular RNAs have been identified whose rates of transcription are altered by incubation of cells with IFNs. The rates of transcription of many of the genes encoding these RNAs are enhanced by IFN-gamma-mediated activation of the Stat1 transcription factor that is tyrosine phosphorylated and translocates to the nucleus, where it binds enhancers present in IFN-stimulated genes (ISGs). IFN-gamma can also modify the concentrations of some RNAs by posttranscriptional mechanisms. However, very little is understood about the molecular mechanisms regulating this phenomenon. We have identified the RNA encoding thrombomodulin (TM), a physiologic receptor for thrombin, that is downregulated in primary human monocytes incubated with IFN-gamma. Using actinomycin D as a transcriptional inhibitor, we show that the mRNA half-life is rapidly shortened by IFN-gamma. The TM transcript contains a large 3'-untranslated region (UTR), with several AU-rich elements (AREs), elements that have been implicated in the regulation of mRNA decay. Using a tetracycline-regulatory promoter system, we analyzed RNA levels in the absence of transcription of TM. Results from these experiments indicate that incubation of cells with IFN-gamma accelerates the decay of TM RNA through its 3'-UTR. This is the first report describing a clear posttranscriptional downregulation of an mRNA by IFN-gamma that identifies the 3'-UTR as a target of IFN-gamma-stimulated destabilization.

Cells previously desensitized to type 1 interferons display different mechanisms of activation of stat-dependent gene expression from naïve cells

Over the past decade, a wealth of knowledge has been obtained concerning the mechanisms by which interferons (IFNs) and other cytokines activate or down-regulate immediate early genes via the Jak/Stat pathway. In contrast, little information is available on interferon-activated gene expression in naïve cells compared with cells that have been desensitized and subsequently resensitized to the actions of these cytokines. In naïve cells, the ISG54 gene is activated via IFN beta-stimulated formation of ISGF3, a heterotrimeric DNA binding complex consisting of p48 (IRF9) and tyrosine-phosphorylated Stat1 and Stat2. In contrast, in previously desensitized cells IFN beta weakly stimulates the assembly of an ISGF3-like complex that lacks Stat1, even though ISG54 mRNA induction is the same as in naïve cells. The lack of Stat1 tyrosine phosphorylation and DNA binding is due to increased activity of a protein-tyrosine phosphatase. In cells that do not express the tyrosine phosphatase Tc-PTP, the rate of Stat1 dephosphorylation is the same in naïve and previously desensitized cells. These results implicate Tc-PTP in a novel role in the regulation of type 1 interferon-stimulated gene expression.

A PI-3 kinase-dependent, Stat1-independent signaling pathway regulates interferon-stimulated monocyte adhesion

Type I interferon (IFN)-alpha/beta and type II IFN-gamma induce the expression of early response genes through activation of the Janus tyrosine kinase/signal transducer and activator of transcription (Stat) pathway. Although IFNs regulate a variety of other signaling cascades, little is known about how they contribute to the biological activities of these cytokines. In this study, we demonstrate that IFN-beta or IFN-gamma induces the phosphorylation of the serine/threonine kinase Akt in primary human peripheral blood monocytes. Abrogation of the IFN-stimulated Akt activation by phosphatidylinositol-3 kinase (PI-3K) inhibitors prevents IFN-induced adhesion in these cells, and IFN activation of the Stat1-dependent guanylate-binding protein (GBP) gene is not affected. Importantly, Stat1-deficient bone marrow macrophages displayed a similar level of IFN-gamma-stimulated adhesion compared with macrophages derived from wild-type littermates. These findings demonstrate for the first time that IFN stimulation of a PI-3K signaling cascade modulates the ability of these cytokines to regulate monocyte adhesion, and this process does not require the expression of Stat1, a primary mediator of IFN-gamma signaling.

The carboxyl terminus of the granulocyte colony-stimulating factor receptor, truncated in patients with severe congenital neutropenia/acute myeloid leukemia, is required for SH2-containing phosphatase-1 suppression of Stat activation

The G-CSF receptor transduces signals that regulate the proliferation, differentiation, and survival of myeloid cells. A subgroup of patients with severe congenital neutropenia (SCN) has been shown to harbor mutations in the G-CSF receptor gene that resulted in the truncation of the receptor's carboxyl-terminal region. SCN patients with mutations in the G-CSF receptor gene are predisposed to acute myeloid leukemia. The truncated receptors from SCN/acute myeloid leukemia patients mediate augmented and sustained activation of Stat transcription factors and are accordingly hyperactive in inducing cell proliferation and survival but are defective in inducing differentiation. Little is known about the molecular mechanisms underlying the negative role of the receptor's carboxyl terminus in the regulation of Stat activation and cell proliferation/survival. In this study, we provide evidence that SH2-containing phosphatase-1 (SHP-1) plays a negative regulatory role in G-CSF-induced Stat activation. We also demonstrate that the carboxyl terminus of the G-CSF receptor is required for SHP-1 down-regulation of Stat activation induced by G-CSF. Our results indicate further that this regulation is highly specific because SHP-1 has no effect on the activation of Akt and extracellular signal-related kinase1/2 by G-CSF. The data together strongly suggest that SHP-1 may represent an important mechanism by which the carboxyl terminus of the G-CSF receptor down-regulates G-CSF-induced Stat activation and thereby inhibits cell proliferation and survival in response to G-CSF.

Vanadate facilitates interferon alpha-mediated apoptosis that is dependent on the Jak/Stat pathway

Type I interferon (IFN)-dependent inhibition of cell growth can occur either in the absence or presence of apoptosis. The mechanisms that determine whether or not cells undergo apoptosis after exposure to IFN-alpha are not clear. This study shows that a variety of cell lines that display growth inhibition but not apoptosis in response to IFN-alpha will undergo programmed cell death when low concentrations of the protein-tyrosine phosphatase inhibitor vanadate are added with IFN-alpha. In contrast, the combination of tumor necrosis factor-alpha with vanadate did not trigger apoptosis in these cells. Caspase-3 activity was detected only in cells exposed to IFN-alpha and vanadate but not to IFN-alpha or vanadate alone. The ability of IFN-alpha and vanadate to induce apoptosis did not require expression of p53 and was blocked by N-acetyl-l-cysteine. Activation of the Jak/Stat pathway and expression of IFN-inducible genes was not altered by incubation of cells with IFN-alpha and vanadate compared with IFN-alpha alone. However, mutant cells lacking Stat1, Stat2, Jak1, or Tyk2, or cells expressing kinase inactive Jak1 or Tyk2 did not undergo apoptosis in the presence of IFN-alpha and vanadate. These results suggest that IFN-alpha stimulation of Stat-dependent genes is necessary, but not sufficient, for this cytokine to induce apoptosis. Another signaling cascade that involves the activity of a protein-tyrosine phosphatase and/or the generation of reactive oxygen species may play an important role in promoting IFN-alpha-induced apoptosis.

Granulocyte colony-stimulating factor induces ERK5 activation, which is differentially regulated by protein-tyrosine kinases and protein kinase C. Regulation of cell proliferation and survival

Granulocyte colony-stimulating factor (G-CSF) plays a major role in the regulation of granulopoiesis. Treatment of cells with G-CSF has been shown to activate multiple signal transduction pathways. We show here that Erk5, a novel member of the MAPK family, and its specific upstream activator MEK5 were activated in response to incubation of cells with G-CSF. Different from other members of the MAPK family including Erk1/2, JNK, and p38, maximal activation of Erk5 by G-CSF required the C-terminal region of the G-CSF receptor. Genistein, a specific inhibitor of protein-tyrosine kinases, blocked G-CSF-induced Erk5 activation. In contrast, inhibition of protein kinase C activity increased G-CSF-mediated activation of Erk5 and MEK5, whereas stimulation of protein kinase C activity inhibited activation of the two kinases by G-CSF. The proliferation of BAF3 cells in response to G-CSF was inhibited by expression of a dominant-negative MEK5 but potentiated by expression of a constitutively active MEK5. Expression of the constitutively active MEK5 also increased the survival of BAF3 cells cultured in the absence of or in low concentrations of G-CSF. Together, these data implicate Erk5 as an important signaling component in the biological actions of G-CSF.

Signaling via the T cell antigen receptor induces phosphorylation of Stat1 on serine 727

The Stat1 transcription factor plays a pivotal role in both, the antiviral and antigrowth actions of interferons. Stat1 acquires the ability to bind DNA by becoming phosphorylated on Tyr(701). However, to effectively stimulate gene transcription, it must also be phosphorylated on Ser(727). We show that engagement of T cell antigen receptor (TCR)/CD3 complex in either Jurkat cells or peripheral blood lymphocytes stimulates phosphorylation of Ser(727) but not Tyr(701) of Stat1. This process does not require the expression of tyrosine kinases Lck and Zap-70. Interestingly, pretreatment of T cells with the Src kinase inhibitor PP1 completely abrogated CD3-mediated serine phosphorylation of Stat1, whereas inhibitors to MEK1 and phosphatidylinositol 3-kinase had no effect. Phosphorylation of Ser(727) of Stat1 in T cells is not restricted to TCR/CD3 but also results when cells are stimulated via the costimulatory molecule CD28. The combination of CD3 and CD28 did not augment phosphorylation of Stat1 Ser(727). Surprisingly, Stat1-mediated transcriptional activity in response to IFN-alpha was enhanced with CD3 stimulation, whereas CD3 alone had little effect. These findings suggest that Stat1 is a signaling molecule in TCR signaling and may play a role in T cell function.

Overlapping and specific functions of Braf and Craf-1 proto-oncogenes during mouse embryogenesis

The three mammalian Raf serine/threonine protein kinases mediate the transduction of proliferative and differentiative signals from cell surface receptors to the nucleus. In vertebrates, Raf signaling has been implicated in the progression of mouse embryos through the two-cell stage and in the induction of posterior mesoderm. However, mouse embryos mutant for each of the Raf genes exhibit no developmental defects before mid-gestation. Here we describe the phenotype of mouse mutants with different combinations of mutant Craf-1 and Braf alleles. Our results show that Raf signaling is indeed indispensable for normal development beyond the blastocyst stage. However, due to a significant redundancy between Craf-1 and Braf, either gene is sufficient for normal development until mid-gestation. The molecular and developmental mechanisms for this redundancy were investigated by monitoring the expression of Raf genes throughout embryogenesis and by biochemical studies in mutant cell lines.

Activation of Akt kinase by granulocyte colony-stimulating factor (G-CSF): evidence for the role of a tyrosine kinase activity distinct from the Janus kinases

Activation of the serine/threonine kinase Akt has been shown to be a critical component for growth factor and cytokine stimulation of cell survival. Although some of the immediate upstream activators of Akt have been defined, the roles of tyrosine kinases in the activation of Akt are not well delineated. Granulocyte colony-stimulating factor (G-CSF) regulates the proliferation, differentiation, and survival of neutrophilic granulocytes. G-CSF exerts its actions by stimulating several signaling cascades after binding its cell surface receptor. Both Jak (Janus) and Src families of tyrosine kinases are stimulated by incubation of cells with G-CSF. In this report, we show that G-CSF stimulation of cells leads to activation of Akt. The membrane-proximal 55 amino acids of the G-CSF receptor cytoplasmic domain are sufficient for mediating Akt activation. However, activation of Akt appears to be downregulated by the receptor's carboxy-terminal region of 98 amino acids, a region that has been shown to be truncated in some patients with acute myeloid leukemia associated with severe congenital neutropenia. Furthermore, we demonstrate that G-CSF-induced activation of Akt requires the activities of Src family kinases but can be clearly dissociated from G-CSF-stimulated activation of Stats (signal transducers and activators of transcription) by the Jak kinases. Thus, cytokine activation of the Jak/Stat and other signaling cascades can be functionally separated. (Blood. 2000;95:1656-1662)

A lymphoma growth inhibitor blocks some but not all prolactin-stimulated signaling pathways

Cytokines and hormones activate a network of intracellular signaling pathways to regulate cell division, survival and differentiation. In parallel, a series of growth inhibitory mechanisms critically restrict cell population sizes. For example, mitogens can be opposed in crowded cell cultures through contact-inhibition or by autocrine release of antiproliferative substances. Here, we characterize a small, heat-stable growth inhibitor secreted by a rat T lymphoma line when cultured at high cell density. Short term incubation (<60 min) of prolactin-responsive Nb2 lymphoma cells at high density selectively blocked prolactin stimulation of p42/p44 mitogen-activated protein kinases and transcription factors Stat1 and Stat3 but not prolactin activation of Stat5 or the tyrosine kinase Jak2. The selective effects of cell density on prolactin signaling were reversible. Furthermore, exposure of cells at low density to conditioned media from cells incubated at high density had the same inhibitory effects on prolactin signaling. This selective inhibition of discrete prolactin signals was mimicked by short term preincubation of cells at low density with staurosporine or genistein but not with bis-indoleyl maleimide, cyclic nucleotide analogs, calcium ionophore A23187, or phorbol 12-myristate 13-acetate. A heat-stable, proteinase K-resistant, low molecular weight factor with these characteristics was recovered from high density culture medium. The partially purified inhibitor suppressed Nb2 cell growth with a sigmoidal concentration response consistent with a saturable, receptor-mediated process.

Interleukin-10 inhibits expression of both interferon alpha- and interferon gamma- induced genes by suppressing tyrosine phosphorylation of STAT1

Interleukin-10 (IL-10) helps maintain polarized T-helper cells in a T-helper lymphocyte 2 (Th2) phenotype. Part of this process involves the prevention of the development of Th1 cells, which are a primary source of interferon gamma (IFNgamma), a potent activator of monocytes and an inhibitor of Th2 proliferation. Because monocytes and macrophages are important mediators of Th1-type responses, such as delayed-type hypersensitivity, we sought to determine if IL-10 could directly mediate inhibition of IFNgamma- and IFNalpha-induced gene expression in these cells. Highly purified monocytes were incubated with IL-10 for 60 to 90 minutes before the addition of IFNgamma or IFNalpha. IL-10 preincubation resulted in the inhibition of gene expression for several IFN-induced genes, such as IP-10, ISG54, and intercellular adhesion molecule-1. The reduction in gene expression resulted from the ability of IL-10 to suppress IFN-induced assembly of signal transducer and activator of transcription (STAT) factors to specific promoter motifs on IFNalpha- and IFNgamma-inducible genes. This was accomplished by preventing the IFN-induced tyrosine phosphorylation of STAT1, a component of both IFNalpha- and IFNgamma-induced DNA binding complexes. Therefore, IL-10 can directly inhibit STAT-dependent early response gene expression induced by both IFNalpha and IFNgamma in monocytes by suppressing the tyrosine phosphorylation of STAT1. This may occur through the ability of IL-10 to induce expression of the gene, suppressor of cytokine signaling 3 (SOCS3).

Craf-1 protein kinase is essential for mouse development

The three mammalian Raf serine/threonine protein kinases mediate the transduction of proliferative and differentiative signals from a variety of cell surface receptors to the nucleus. We report here that Craf-1 is essential for mouse development, as its mutation results in embryonic lethality. Developmental defects are found in mutant placentas as well as in the skin and in the lungs of mutant embryos. Craf-1 mutants also display a generalized growth retardation which is consistent with the ubiquitous expression of Craf-1 and which could be due to the reduced proliferation of mutant cells. Interestingly, the time-point of embryonal death varies depending on the genetic background. This suggests that Craf-1-mediated signaling is affected by genetic background-specific alleles of other genes.

Activation of Raf-1 by interferon gamma and oncostatin M requires expression of the Stat1 transcription factor

A primary signaling cascade responsible for the expression of cytokine-stimulated immediate early genes involves the activation of the Jak/Stat pathway. In addition to being tyrosine-phosphorylated, several signal transducers and activators of transcription (Stats), including Stat1alpha, Stat3, and Stat4, are phosphorylated on a conserved serine residue, which is a consensus phosphorylation site for mitogen-activated protein kinases (MAPKs). Serine phosphorylation of Stat1alpha is required for maximal transcriptional activation of early response genes by interferon gamma (IFNgamma) as well as the antiviral and antigrowth actions of this cytokine. Incubation of cells with either IFNgamma or oncostatin M (OSM) activates Raf-1, a serine/threonine kinase responsible for the ultimate activation of p42 MAPK. To examine whether any of the signaling components that are required for activation of the Jak/Stat pathway are also necessary for activation of Raf-1 by IFNs and OSM, we examined activation of Raf-1 in cell lines that are deficient in either Stat1alpha or Stat2. Unexpectedly, incubation of Stat1-deficient, but not Stat2-deficient cells with IFNgamma or OSM for 5 min displayed no increase in Raf-1 activity. In peripheral blood lymphocytes Raf-1 was associated with Stat1, and this interaction was disrupted after incubation of cells with IFNgamma. Stat1-negative cells reconstituted with either Stat1alpha or Stat1alpha with a point mutation in the site where it is serine-phosphorylated displayed normal activation of Raf-1 by IFNgamma and OSM. However, activation of Raf-1 was not observed in lines that expressed Stat1alpha containing a mutation in its tyrosine phosphorylation site or in its SH2 domain. These results provide the first example of a novel role of Stat1alpha not as a transcription factor, but as a protein which may function to scaffold signaling components required for activation of the distinct Raf/MEK/MAPK signaling cascade.

Control of lytic function by mitogen-activated protein kinase/extracellular regulatory kinase 2 (ERK2) in a human natural killer cell line: identification of perforin and granzyme B mobilization by functional ERK2

The signal pathways that control effector function in human natural killer (NK) cells are little known. In this study, we have identified the critical role of the mitogen-activated protein kinase (MAPK) pathway in NK lysis of tumor cells, and this pathway may involve the mobilization of granule components in NK cells upon interaction with sensitive tumor target cells. Evidence was provided by biological, biochemical, and gene transfection methods. NK cell binding to tumor cells for 5 min was sufficient to maximally activate MAPK/extracellular signal-regulatory kinase 2 (ERK2), demonstrated by its tyrosine phosphorylation and by its ability to function as an efficient kinase for myelin basic protein. MAPK activation was achieved in NK cells only after contact with NK-sensitive but not NK-resistant target cells. In immunocytochemical studies, cytoplasmic perforin and granzyme B were both maximally redirected towards the tumor contact zone within 5 min of NK cell contact with tumor cells. A specific MAPK pathway inhibitor, PD098059, could block not only MAPK activation but also redistribution of perforin/granzyme B in NK cells, which occur upon target ligation. PD098059 also interfered with NK lysis of tumor cells in a 5-h 51Cr-release assay, but had no ability to block NK cell proliferation. Transient transfection studies with wild-type and dominant-negative MAPK/ERK2 genes confirmed the importance of MAPK in NK cell lysis. These results document a pivotal role of MAPK in NK effector function, possibly by its control of movement of lytic granules, and clearly define MAPK involvement in a functional pathway unlinked to cell growth or differentiation.

Cutting Edge: Involvement of SHP-2 in Multiple Aspects of IL-2 Signaling: Evidence for a Positive Regulatory Role

Binding of IL-2 to its receptor activates several biochemical pathways, but precisely how these pathways are linked is incompletely understood. Here, we report that SHP-2, an SH2-domain containing tyrosine phosphatase, associates with different molecules of the IL-2 signaling cascade. Upon IL-2 stimulation, SHP-2 was coimmunoprecipitated with Grb2 and the p85 subunit of phosphatidylinositol 3-kinase. In contrast, SHP-2 was constitutively associated with JAK1 and JAK3. Finally, SHP-2 expression amplified STAT-dependent transcriptional activation whereas a dominant negative allele inhibited transactivation and the IL-2-induced activation of MAPK (mitogen-activated protein kinase). These results demonstrate the involvement of SHP-2 in multiple pathways of the IL-2 signaling cascade and provide evidence for its positive regulatory role.

Involvement of SHP-2 in multiple aspects of IL-2 signaling: evidence for a positive regulatory role

Binding of IL-2 to its receptor activates several biochemical pathways, but precisely how these pathways are linked is incompletely understood. Here, we report that SHP-2, an SH2-domain containing tyrosine phosphatase, associates with different molecules of the IL-2 signaling cascade. Upon IL-2 stimulation, SHP-2 was coimmunoprecipitated with Grb2 and the p85 subunit of phosphatidylinositol 3-kinase. In contrast, SHP-2 was constitutively associated with JAK1 and JAK3. Finally, SHP-2 expression amplified STAT-dependent transcriptional activation whereas a dominant negative allele inhibited transactivation and the IL-2-induced activation of MAPK (mitogen-activated protein kinase). These results demonstrate the involvement of SHP-2 in multiple pathways of the IL-2 signaling cascade and provide evidence for its positive regulatory role.

Interferon gamma activation of Raf-1 is Jak1-dependent and p21ras-independent

Signal transduction through the interferongamma (IFNgamma) receptor involves the formation of a ligand-dependent multimolecular association of receptor chains (alpha and beta), Janus tyrosine kinases (Jak1 and Jak2), and the transcription factor (signal transducers and activators of transcription 1alpha (STAT1alpha)) in addition to activation of mitogen-activated protein kinases (MAPK). Interactions between components of the Jak/STAT cascade and the p21(ras)/Raf-1/MAPK cascade are unexplored. Treatment of HeLa cells with IFNgamma resulted in the rapid and transient activation of Raf-1 and MAPK. Parallel activation of cells resulted in essentially no enhancement of p21(ras) activation despite marked enhancement after treatment with epidermal growth factor. In HeLa (E1C3) and fibrosarcoma (U4A) cell lines, both of which are deficient in Jak1 kinase, Raf-1 activation by IFNgamma was absent. Reconstitution of Raf-1 activity was observed only with kinase active Jak1 in both cell lines. In COS cells, transient expression of wild type or kinase-inactive Jak1 coimmunoprecipitated with Raf-1, but activation of Raf-1 activity was only observed in cells expressing kinase-active Jak1. These observations suggest that a kinase-active Jak1 is required for IFNgamma activation of Raf-1 that is p21(ras)-independent.

Antiproliferative action of interferon-alpha requires components of T-cell-receptor signalling

Signal transduction through both cytokine and lymphocyte antigen receptors shares some common pathways by which they initiate cellular responses, such as activation of mitogen-activated protein kinase(s). However, other signalling components appear to be uniquely coupled to each receptor. For example, the interferon receptors transduce regulatory signals through the JAK/STAT pathway, resulting in an inhibition of growth and of antiviral effects, whereas this pathway apparently plays no role in T-cell-receptor (TCR)-dependent gene expression. Conversely, signal transduction through the TCR requires the tyrosine kinases Lck and ZAP-70 and the tyrosine phosphatase CD45. Here we show that, unexpectedly, transmission of growth-inhibitory signals by interferon-alpha (IFN-alpha) in T cells requires the expression and association of CD45, Lck and ZAP-70 with the IFN-alpha-receptor signalling complex.

Beta interferon and oncostatin M activate Raf-1 and mitogen-activated protein kinase through a JAK1-dependent pathway

Activation of early response genes by interferons (IFNs) and other cytokines requires tyrosine phosphorylation of a family of transcription factors termed signal transducers and activators of transcription (Stats). The Janus family of tyrosine kinases (Jak1, Jak2, Jak3, and Tyk2) is required for cytokine-induced tyrosine phosphorylation and dimerization of the Stat proteins. In order for IFNs to stimulate maximal expression of Stat1alpha-regulated genes, phosphorylation of a serine residue in the carboxy terminus by mitogen-activated protein kinase (MAPK) is also required. In HeLa cells, both IFN-beta and oncostatin M (OSM) stimulated MAPK and Raf-1 enzyme activity, in addition to Stat1 and Stat3 tyrosine phosphorylation. OSM stimulation of Raf-1 correlated with GTP loading of Ras, whereas IFN-beta activation of Raf-1 was Ras independent. IFN-beta- and OSM-induced Raf-1 activity could be coimmunoprecipitated with either Jak1 or Tyk2. Furthermore, HeLa cells lacking Jak1 displayed no activation of STAT1alpha, STAT3, and Raf-1 by IFN-beta or OSM and also demonstrated no increase in the relative level of GTP-bound p21ras in response to OSM. The requirement for Jak1 for IFN-beta- and OSM-induced activation of Raf-1 was also seen in Jak1-deficient U4A fibrosarcoma cells. Interestingly, basal MAPK, but not Raf-1, activity was constitutively enhanced in Jak1-deficient HeLa cells. Transient expression of Jak1 in both Jak-deficient HeLa cells and U4A cells reconstituted the ability of IFN-beta and OSM to activate Raf-1 and decreased the basal activity of MAPK, while expression of a kinase-inactive form of the protein showed no effect. Moreover, U4A cells selected for stable expression of Jak1, or COS cells transiently expressing Jak1 or Tyk2 but not Jak3, exhibited enhanced Raf-1 activity. Therefore, it appears that Jak1 is required for Raf-1 activation by both IFN-beta and OSM. These results provide evidence for a link between the Jaks and the Raf/MAPK signaling pathways.

Prolactin stimulates serine/tyrosine phosphorylation and formation of heterocomplexes of multiple Stat5 isoforms in Nb2 lymphocytes

Transcription factors of the Stat gene family are selectively activated by many hormones and cytokines. Stat5 originally was cloned as a prolactin-stimulated DNA-binding protein, but is also activated by non-lactogenic cytokines in many cell types. The recent identification of two distinct Stat5 genes, which encode a 94-kDa Stat5a and a 92-kDa Stat5b as well as several lower molecular weight isoforms, suggests additional complexity and combinatorial possibilities for transcriptional regulation. We now report a biochemical analysis of prolactin activation of Stat proteins in Nb2 lymphocytes, which was associated with: 1) rapid tyrosine phosphorylation of Stat5a, Stat5b, a COOH-terminally truncated 80-kDa Stat5 form, Stat1alpha, and Stat3; 2) rapid and selective formation of Stat5a/b heterodimers, without involvement of Stat1alpha or Stat3; 3) marked serine, but not threonine phosphorylation of Stat5a and Stat5b; and 4) the appearance of two qualitatively distinct Stat5 protein complexes, which discriminated between oligonucleotides corresponding to the prolactin response elements of the beta-casein and interferon regulatory factor-1 gene promoters. Collectively, our analyses showed that Stat5a and Stat5b respond similarly to prolactin receptor activation, but also suggested that the two genes have evolved unique properties that may contribute to the specificity of receptors that utilize Stat5 signaling proteins.

Mapping of a cytoplasmic domain of the human growth hormone receptor that regulates rates of inactivation of Jak2 and Stat proteins

It has been previously demonstrated that growth hormone (GH)-stimulated tyrosine phosphorylation of Jak2 and Stat5a and Stat5b occurs in FDP-C1 cells expressing either the entire GH receptor or truncations of the cytoplasmic domain expressing only the membrane-proximal 80 amino acids. However, other receptor domains that might modulate rates of GH activation and inactivation of this cascade have not been examined. Here we have defined a region in the human GH receptor between amino acids 520 and 540 in the cytoplasmic domain that is required for attenuation of GH-activated Jak/Stat signaling. Immunoprecipitations with antibodies to Jak2 indicate that the protein tyrosine phosphatase SHP-1 is associated with this kinase in cells exposed to GH. To address the possibility that SHP-1 could function as a negative regulator of GH signaling, liver extracts from motheaten mice deficient in SHP-1 or unaffected littermates were analyzed for activation of Stats and Jak2. Extracts from motheaten mice displayed prolonged activation of the Stat proteins as measured by their ability to interact with DNA and prolonged tyrosine phosphorylation of Jak2. These results delineate a novel domain in the GH receptor that regulates the inactivation of the Jak/Stat pathway and appears to be modulated by SHP-1.

Phosphorylation events modulate the ability of interferon consensus sequence binding protein to interact with interferon regulatory factors and to bind DNA

Two families of transcription factors mediate interferon (IFN) signaling. The first family, signal transducers and activators of transcription (STATs), is activated within minutes of IFN treatment. Specific phosphorylation events lead to their translocation to the nucleus, formation of transcriptional complexes, and the induction of the second family of transcription factors termed interferon regulatory factors (IRFs). Interferon consensus sequence binding protein (ICSBP) is a member of IRF family that is expressed only in cells of the immune system and acts as a transcriptional repressor. ICSBP binds DNA through the association with other transcription factors such as IRF-1 or IRF-2. In this communication, the domain that is involved in protein-protein interactions was mapped to the carboxyl terminus of ICSBP. This domain is also important for mediating ICSBP-repressing activity. In vitro studies demonstrated that direct binding of ICSBP to DNA is prevented by tyrosine (Tyr) phosphorylation. Yet, Tyr-phosphorylated ICSBP can bind target DNA only through the association with IRF-2 and IRF-1. This type of phosphorylation is essential for the formation of heterocomplexes. Tyr-phosphorylated ICSBP and IRF-2 are detected in expressing cells constitutively, and Tyr-phosphorylated IRF-1 is induced by IFN-gamma. These results strongly suggest that like the STATs, the IRFs are also modulated by Tyr phosphorylation that affects their biological activities.

The SH2 domain-containing tyrosine phosphatase PTP1D is required for interferon alpha/beta-induced gene expression

Interferons (IFNs) induce early response genes by stimulating Janus family (Jak) tyrosine kinases, leading to tyrosine phosphorylation of Stat (signal transducer and activator of transcription) proteins. Previous studies demonstrated that a protein-tyrosine phosphatase (PTP) is required for activation of the ISGF3 transcription complex by IFNalpha/beta, but the specific PTP responsible remained unidentified. We now show that the SH2 domain containing tyrosine phosphatase PTP1D (also designated as SHPTP2, SHPTP3, PTP2C, or Syp) is constitutively associated with the IFNalpha/beta receptor and becomes tyrosine-phosphorylated in response to ligand. Furthermore, transient expression of a phosphatase-inactive mutant or the COOH-terminal SH2 domain of PTP1D causes a dominant negative effect on IFNalpha/beta-induced early response gene expression. These results provide strong evidence that PTP1D functions as a positive regulator of the IFNalpha/beta-induced Jak/Stat signal transduction pathway.

STAT activation by epidermal growth factor (EGF) and amphiregulin. Requirement for the EGF receptor kinase but not for tyrosine phosphorylation sites or JAK1

The epidermal growth factor (EGF) receptor activates several signaling cascades in response to the ligands EGF and amphiregulin (AR). One of these signaling events involves the tyrosine phosphorylation of STATs (signal transducers and activators of transcription), a process believed to require the activation of a tyrosine kinase of the JAK family. In this report we demonstrate that EGF- and AR-induced STAT activation requires the intrinsic kinase activity of the receptor but not the presence of Jak1. We show that both wild type (WT) and truncated EGF receptors lacking all autophosphorylation sites activate STAT 1, 3, and 5 in response to either EGF or AR. Furthermore, relative to cells expressing WT receptor, ligand-induced tyrosine phosphorylation of the STATs was enhanced in cells expressing only the truncated receptor. These results provide the first evidence that (i) EGF receptor-mediated STAT activation occurs in a Jak1-independent manner, (ii) the intrinsic tyrosine kinase activity of the receptor is essential for STAT activation, and (iii) tyrosine phosphorylation sites within the EGF receptor are not required for STAT activation.

Inhibition of alpha interferon but not gamma interferon signal transduction by phorbol esters is mediated by a tyrosine phosphatase

Previous studies have indicated that the expression of viral oncoproteins, cell transformation, or phorbol ester treatment of cells can inhibit alpha/beta interferon (IFN-alpha/beta)-induced gene expression. The mechanisms by which these promoters of cell growth exert their inhibitory effects vary, but in most instances they involve a disruption of the IFN-alpha/beta-induced transcription complex ISGF3 such that the DNA-binding component of this complex (the 48-kDa ISGF3gamma protein) does not bind to the interferon-stimulated response element (ISRE). In this report, we demonstrated that phorbol ester treatment of human peripheral blood monocytes dramatically inhibits activation of IFN-alpha/B-stimulated early response genes but by a mechanism which does not involve abrogation of the ISRE binding of ISGF3gamma. Phorbol ester treatment of monocytes inhibited IFN alpha-stimulated tyrosine phosphorylation of the transcription factors Stat1alpha, Stat2, and Stat3 and of the tyrosine kinase Tyk2 but had no effect on IFN-gamma activation of Stat1alpha. IFNalpha-stimulated tyrosine phosphorylation of Jak1 and the alpha subunit of the IFN-alpha receptor were unaffected by phorbol 12-myristate 13-acetate (PMA). Moreover, PMA caused the dephosphorylation of Tyk2 but not of Jak1, which was activated by IFN. Pretreatment of cells with vanadate prevented the effects of PMA with regard to PMA-induced Tyk2 dephosphorylation. These observations suggest that PMA exerts its inhibitory effects by activation of a tyrosine phosphatase which selectively regulates Tyk2 but not Jak1 activity.

Prolactin recruits STAT1, STAT3 and STAT5 independent of conserved receptor tyrosines TYR402, TYR479, TYR515 and TYR580

The present study of prolactin (PRL) receptor-mediated recruitment of signal transducers and activators of transcription (STATs) demonstrates that PRL activates STAT3, in addition to STAT1 and STAT5 as previously reported, and that STAT1, STAT3 and STAT5 are mediators of PRL effects in cells whether of lymphoid, myeloid or mammary epithelial origin. Furthermore, receptor mutants M240 and T280 that do not mediate PRL-induced JAK2 activation and cell proliferation, are also unable to mediate STAT activation, supporting the proposed model of JAK2 as the initial effector protein used by PRL receptors. On the other hand, tyrosine phosphorylation analysis and electrophoretic mobility shift assays showed that receptor mutant G328, which lacks four of the five conserved cytoplasmic tyrosine residues of PRL receptors, retained the ability to activate JAK2 and STAT1, STAT3 and STAT5. These results support the notion that phosphotyrosyl residues other than those of the receptor, i.e., JAK2, are involved in recruiting STAT proteins to the activated PRL receptor complex.

Activation of protein kinase A inhibits interferon induction of the Jak/Stat pathway in U266 cells

Activation of early response genes by interferons (IFNs) requires tyrosine phosphorylation of the Stat transcription factors and is mediated by the Jak family of tyrosine kinases. Recent evidence suggests that ERK2 serine/threonine kinase modulates the IFN-stimulated Jak/Stat pathway. In this report we show that in the myeloma cell line U266 protein kinase A specifically interacts with the cytoplasmic domain of the IFNalpha/beta receptor. Treatment of cells with the adenylate cyclase activator forskolin inhibits IFNbeta-, IFNgamma-, and hydrogen peroxide/vanadate-induced formation of complexes that bind to enhancers known to stimulate the expression of IFN-regulated genes. Immunoprecipitations followed by anti-phosphotyrosine immunoblots indicate that tyrosine phosphorylation of the alpha chain of the IFNalpha/beta receptor, Jak1, Tyk2, as well as Stat1 and Stat2 is reduced as a consequence of incubation of cells with forskolin. In contrast, dideoxyforskolin, which fails to activate adenylate cyclase, has no effect on IFN induction of the Jak/Stat pathway. These results indicate a novel regulatory mechanism by which protein kinase A can modulate the Jak/Stat signaling cascade.