Complementation by the protein tyrosine kinase JAK2 of a mutant cell line defective in the interferon-gamma signal transduction pathway

The mEPN scheme: an intuitive and flexible graphical system for rendering biological pathways

Background

There is general agreement amongst biologists about the need for good pathway diagrams and a need to formalize the way biological pathways are depicted. However, implementing and agreeing how best to do this is currently the subject of some debate.

Results

The modified Edinburgh Pathway Notation (mEPN) scheme is founded on a notation system originally devised a number of years ago and through use has now been refined extensively. This process has been primarily driven by the author's attempts to produce process diagrams for a diverse range of biological pathways, particularly with respect to immune signaling in mammals. Here we provide a specification of the mEPN notation, its symbols, rules for its use and a comparison to the proposed Systems Biology Graphical Notation (SBGN) scheme.

Conclusions

We hope this work will contribute to the on-going community effort to develop a standard for depicting pathways and will provide a coherent guide to those planning to construct pathway diagrams of their biological systems of interest.

Genome-wide expression profiling in the Drosophila eye reveals unexpected repression of notch signaling by the JAK/STAT pathway

Although the JAK/STAT pathway regulates numerous processes in vertebrates and invertebrates through modulating transcription, its functionally relevant transcriptional targets remain largely unknown. With one jak and one stat (stat92E), Drosophila provides a powerful system for finding new JAK/STAT target genes. Genome-wide expression profiling on eye discs in which Stat92E is hyperactivated, revealed 584 differentially regulated genes, including known targets domeless, socs36E, and wingless. Other differentially regulated genes (chinmo, lama, Mo25, Imp-L2, Serrate, Delta) were validated and may represent new Stat92E targets. Genetic experiments revealed that Stat92E cell-autonomously represses Serrate, which encodes a Notch ligand. Loss of Stat92E led to de-repression of Serrate in the dorsal eye, resulting in ectopic Notch signaling and aberrant eye growth there. Thus, our micro-array documents a new Stat92E target gene and a previously unidentified inhibitory action of Stat92E on Notch signaling. These data suggest that this study will be a useful resource for the identification of additional Stat92E targets.

JAK2 phosphorylates histone H3Y41 and excludes HP1alpha from chromatin

Activation of Janus kinase 2 (JAK2) by chromosomal translocations or point mutations is a frequent event in haematological malignancies. JAK2 is a non-receptor tyrosine kinase that regulates several cellular processes by inducing cytoplasmic signalling cascades. Here we show that human JAK2 is present in the nucleus of haematopoietic cells and directly phosphorylates Tyr 41 (Y41) on histone H3. Heterochromatin protein 1alpha (HP1alpha), but not HP1beta, specifically binds to this region of H3 through its chromo-shadow domain. Phosphorylation of H3Y41 by JAK2 prevents this binding. Inhibition of JAK2 activity in human leukaemic cells decreases both the expression of the haematopoietic oncogene lmo2 and the phosphorylation of H3Y41 at its promoter, while simultaneously increasing the binding of HP1alpha at the same site. Tauhese results identify a previously unrecognized nuclear role for JAK2 in the phosphorylation of H3Y41 and reveal a direct mechanistic link between two genes, jak2 and lmo2, involved in normal haematopoiesis and leukaemia.

An evolutionary conserved function of the JAK-STAT pathway in anti-dengue defense

Here, we show that the major mosquito vector for dengue virus uses the JAK-STAT pathway to control virus infection. Dengue virus infection in Aedes aegypti mosquitoes activates the JAK-STAT immune signaling pathway. The mosquito's susceptibility to dengue virus infection increases when the JAK-STAT pathway is suppressed through RNAi depletion of its receptor Domeless (Dome) and the Janus kinase (Hop), whereas mosquitoes become more resistant to the virus when the negative regulator of the JAK-STAT pathway, PIAS, is silenced. The JAK-STAT pathway exerts its anti-dengue activity presumably through one or several STAT-regulated effectors. We have identified, and partially characterized, two JAK-STAT pathway-regulated and infection-responsive dengue virus restriction factors (DVRFs) that contain putative STAT-binding sites in their promoter regions. Our data suggest that the JAK-STAT pathway is part of the A. aegypti mosquito's anti-dengue defense and may act independently of the Toll pathway and the RNAi-mediated antiviral defenses.

MS-1020 is a novel small molecule that selectively inhibits JAK3 activity

In order to identify Janus kinase/signal transducer and activator of transcription (JAK/STAT) signalling inhibitors, a cell-based high throughput screening was performed using a plant extract library that identified Nb-(alpha-hydroxynaphthoyl)serotonin called MS-1020 as a novel JAK3 inhibitor. MS-1020 potently inhibited persistently-active STAT3 in a cell type-specific manner. Further examination showed that MS-1020 selectively blocked constitutively-active JAK3 and consistently suppressed interleukin-2-induced JAK3/STAT5 signalling but not prolactin-induced JAK2/STAT5 signalling. Furthermore, MS-1020 affected cell viability only in cancer cells harbouring persistently-active JAK3/STATs, and in vitro kinase assays showed MS-1020 binds directly with JAK3, blocking its catalytic activity. Therefore, the present study suggested that this reagent selectively inhibits JAK3 and subsequently leads to a block in STAT signalling. Finally, MS-1020 decreased cell survival by inducing apoptosis via down-regulation of anti-apoptotic gene expression. These results suggest that MS-1020 may have therapeutic potential in the treatment of cancers harbouring aberrant JAK3 signalling.

The C-terminus of interferon gamma receptor beta chain (IFNgammaR2) has antiapoptotic activity as a Bax inhibitor

Bax is a pro-apoptotic protein that mediates intrinsic cell-death signaling. Using a yeast-based functional screening approach, we identified interferon gamma receptor beta chain (IFNgammaR2) as a new Bax suppressor. IFNgammaR2 is a component of the IFNgamma receptor complex along with the IFNgammaR alpha chain (IFNgammaR1). Upon IFNgamma binding, a conformational change in the receptor complex occurs that activates the Jak2/STAT1 signaling cascade. We found that the C-terminal region (amino acids 296-337) of IFNgammaR2 (IFNgammaR2(296-337)) contains a novel Bax inhibitory domain. This portion does not contain the Jak2-binding domain; therefore, the antiapoptotic function of IFNgammaR2 is independent of JAK/STAT signaling. IFNgammaR2(296-337) rescued human cells from apoptosis induced by overexpression of Bax but not Bak. Overexpression of IFNgammaR2 (wild type and IFNgammaR2(296-337)) rescued cells from etoposide and staurosporine, which are known to induce Bax-mediated cell death. Interestingly, IFNgammaR2 inhibited apoptosis induced by the BH3-only protein Bim-EL, suggesting that IFNgammaR2 inhibits Bax activation through a BH3-only protein. Bax and IFNgammaR2 were co-immunoprecipitated from cell lysates prepared from HEK293 and DAMI cells. Furthermore, direct binding of purified recombinant proteins of Bax and IFNgammaR2 was also confirmed. Addition of recombinant Bcl-2 protein to cell lysates significantly reduced the interaction of IFNgammaR2 and Bax, suggesting that Bcl-2 and IFNgammaR2 bind a similar domain of Bax. We found that the C-terminal fragment (cytoplasmic domain) of IFNgammaR2 is expressed in human cancer cell lines of megakaryocytic cancer (DAMI), breast cancer (MDA-MD-468), and prostate cancer (PC3 cells). The presence of the C-terminal fragment of IFNgammaR2 may confer on cancer cells resistance to apoptotic stresses. Our discovery of the anti-Bax activity of the cytoplasmic domain of IFNgammaR2 may shed new light on the mechanism of how cell death is controlled by IFNgamma and Bax.

Real-time bioluminescence imaging of polycythemia vera development in mice

Polycythemia vera (PV) is a myeloproliferative disorder involving hematopoietic stem cells. A recurrent somatic missense mutation in JAK2 (JAK2V617F) is thought to play a causal role in PV. Therefore, targeting Jak2 will likely provide a molecular mechanism-based therapy for PV. To facilitate the development of such new and specific therapeutics, a suitable and well-characterized preclinical animal model is essential. Although several mouse models of PV have been reported, the spatiotemporal kinetics of PV formation and progression has not been studied. To address this, we created a bone marrow transplant mouse model that co-expresses mutant Jak2 and luciferase 2 (Luc2) genes. Bioluminescent imaging (BLI) was used to visualize disease cells and analyze the kinetics of PV development in vivo. To better understand the molecular mechanism of PV, we generated mice carrying a kinase inactive mutant Jak2 (Jak2K882E), demonstrating that the PV disease was dependent on constitutive activation of the Jak2 kinase activity. We further showed that the Jak2V617F mutation caused increased stem cell renewal activity and impaired cell differentiation, which was at least in part due to deregulated transcriptional programming. The Jak2V617F-Luc2 PV mice will be a useful preclinical model to characterize novel JAK2 inhibitors for the treatment of PV.

An unusual insertion in Jak2 is crucial for kinase activity and differentially affects cytokine responses

The Janus kinases, Jaks, constitutively associate with the cytoplasmic region of cytokine receptors and play an important role in a multitude of biological processes. Jak2 dysfunction has been implicated in myeloproliferative diseases and leukemia. Although Jaks were studied extensively for many years, the molecular mechanism of Jak activation upon cytokine stimulation of cells is still incompletely understood. In this study, we investigated the importance of an unusual insertion located within the kinase domain in Jak2. We found that the deletion of this insertion, which we named the Jak-specific insertion (JSI), totally abrogates Jak2 autophosphorylation. We further point mutated four residues within the JSI that are conserved in all Jak family members. Three of these mutants showed abrogated or reduced autophosphorylation, whereas the fourth displayed increased autophosphorylation. We found that the phosphorylation state of these mutants is not influenced by other domains of the kinase. Our data further suggest that the JSI is not required for the negative regulation of kinase activity by the suppressor of cytokine signaling proteins, SOCS. Most importantly, we show that mutations in this region differentially affect IFN-gamma and erythropoietin signal transduction. Taken together, the dramatic effects on the phosphorylation status of Jak2 as well as the differential effects on the signaling via different cytokines highlight the importance of this unusual region for the catalytic activity of Jaks.

Janus kinases in immune cell signaling

The Janus family kinases (Jaks), Jak1, Jak2, Jak3, and Tyk2, form one subgroup of the non-receptor protein tyrosine kinases. They are involved in cell growth, survival, development, and differentiation of a variety of cells but are critically important for immune cells and hematopoietic cells. Data from experimental mice and clinical observations have unraveled multiple signaling events mediated by Jaks in innate and adaptive immunity. Deficiency of Jak3 or Tyk2 results in defined clinical disorders, which are also evident in mouse models. A striking phenotype associated with inactivating Jak3 mutations is severe combined immunodeficiency syndrome, whereas mutation of Tyk2 results in another primary immunodeficiency termed autosomal recessive hyperimmunoglobulin E syndrome. By contrast, complete deletion of Jak1 or Jak2 in the mouse are not compatible with life and, unsurprisingly, do not have counterparts in human disease. However, activating mutations of each of the Jaks are found in association with malignant transformation, the most common being gain-of-function mutations of Jak2 in polycythemia vera and other myeloproliferative disorders. Our existing knowledge on Jak signaling pathways and fundamental work on their biochemical structure and intracellular interactions allow us to develop new strategies for controlling autoimmune diseases or malignancies by developing selective Jak inhibitors, which are now coming into clinical use. Despite the fact that Jaks were discovered only a little more than a decade ago, at the time of writing there are 20 clinical trials underway testing the safety and efficacy of Jak inhibitors.

Interferon gamma enhances clonal expansion and survival of CD4+ T cells

Interferon-gamma (IFN-gamma) serves numerous functions in the regulation of the immune response. During the early phase of the immune response IFN-gamma is produced by natural killer and natural killer T cells. Although the effects of this cytokine on antigen presenting cells and other cell types are known, its direct role on CD4(+) T cells remains unclear. We demonstrate that CD4(+) T cells exposed to IFN-gamma proliferate more vigorously than the controls in response to signals through the antigen receptor. The increased proliferation of IFN-gamma-treated CD4(+) T cells is not due to enhanced signaling through the antigen receptor, but is accounted for by their increased survival. Our data suggest that enhanced survival of IFN-gamma-treated CD4(+)T cells is independent of signal transducer and activator of transcription 1 (STAT 1), a transcription factor that controls the expression of a variety of IFN-gamma-targeted genes. In addition, we demonstrate that independent of STAT 1, IFN-gamma treatment increases the expression of double-stranded RNA-dependent protein kinase, a kinase involved in regulating protein synthesis. Taken together, our findings suggest a direct role of IFN-gamma on unstimulated CD4(+) T cells that is likely to enhance the advent of adaptive immunity by augmenting their survival during the initiation of the immune response.

Hepcidin-induced internalization of ferroportin requires binding and cooperative interaction with Jak2

Hepcidin is a hormone secreted in response to iron loading and inflammation. Hepcidin binds to the iron exporter ferroportin, inducing its degradation and thus preventing iron entry into plasma. We determined that hepcidin binding to ferroportin leads to the binding and activation of the protein Janus Kinase2 (Jak2), which is required for phosphorylation of ferroportin. Ferroportin is a dimer and both monomers must be capable of binding hepcidin for Jak2 to bind to ferroportin. Once Jak2 is bound to the ferroportin dimer, both ferroportin monomers must be functionally competent to activate Jak2 and for ferroportin to be phosphorylated. These results show that cooperativity between the ferroportin monomers is required for hepcidin-mediated Jak2 activation and ferroportin down-regulation. These results provide a molecular explanation for the dominant inheritance of hepcidin resistant iron overload disease.

Janus kinases promote cell-surface expression and provoke autonomous signalling from routing-defective G-CSF receptors

CSF3R [G-CSF (granulocyte colony-stimulating factor) receptor] controls survival, proliferation and differentiation of myeloid progenitor cells via activation of multiple JAKs (Janus kinases). In addition to their role in phosphorylation of receptor tyrosine residues and downstream signalling substrates, JAKs have recently been implicated in controlling expression of cytokine receptors, predominantly by masking critical motifs involved in endocytosis and lysosomal targeting. In the present study, we show that increasing the levels of JAK1, JAK2 and TYK2 (tyrosine kinase 2) elevated steady-state CSF3R cell-surface expression and enhanced CSF3R protein stability in haematopoietic cells. This effect was not due to inhibition of endocytotic routing, since JAKs did not functionally interfere with the dileucine-based internalization motif or lysine-mediated lysosomal degradation of CSF3R. Rather, JAKs appeared to act on CSF3R in the biosynthetic pathway at the level of the ER (endoplasmic reticulum). Strikingly, increased JAK levels synergized with internalization- or lysosomal-routing-defective CSF3R mutants to confer growth-factor independent STAT3 (signal transducer and activator of transcription 3) activation and cell survival, providing a model for how increased JAK expression and disturbed intracellular routing of CSF3R synergize in the transformation of haematopoietic cells.

A study of death by anoikis in cultured epithelial cells

BACKGROUND

Epithelial cells are critically dependent upon cell-matrix and cell-cell adhesion for growth and survival. Anoikis is programmed cell death caused by disruption of cell-substrate adhesion in normal epithelial cells.

METHODS

We studied the induction of anoikis in vitro in two cell lines; HaCaT and SW742. PI3K, JAK2 and PKC are key elements in signalling pathways regulating cell survival, and using specific inhibitors we also examined their potential role in the induction of anoikis.

RESULTS

When prevented from adhesion by culture on polyHEMA, HaCaT cells underwent apoptosis selectively from the proliferating population; surviving cells underwent cell cycle arrest. In SW742 cells anoikis also occurred, but was balanced by increased cycling. The effects of specific kinase inhibitors indicated that both Janus kinase 2 and protein kinase C partially protect HaCaT cells from anoikis through inducing cell cycle arrest of surviving nonadherent cells; inhibition of Phosphatidylinositol 3-kinase did not induce cycling in HaCaTs prevented from adhesion but did stimulate anoikis. SW742 cells showed markedly different responses: Janus kinase 2 inhibition activated apoptosis directly, Phosphatidylinositol 3-kinase inhibition stimulated both cell cycling and apoptosis, while protein kinase C inhibition stimulated cycling but inhibited apoptosis.

CONCLUSIONS

Susceptibility to cell death in adhesion-prevented epithelial cells may thus be regulated by signalling pathways involving Phosphatidylinositol 3-kinase, Janus kinase 2 and protein kinase C. The ability of epithelial tumour cells to invade and metastasize may therefore result from disruption of these pathways.

Inteferons pen the JAK-STAT pathway

Characterization of how interferons (IFNs) mediate their biological response led to identification of the JAK-STAT signaling cascade, where JAKs are receptor-associated kinases and STATs the transcription factors they activate. Today, 4 JAKs and 7 STATs are known to transduce pivotal signals for the over 50 members of the four-helix bundle family of cytokines. This review will provide an overview and historical perspective of the JAK-STAT paradigm.

JAKs in pathology: role of Janus kinases in hematopoietic malignancies and immunodeficiencies

The four mammalian Janus kinase (JAK) family members, JAK1, JAK2, JAK3 and TYK2, are non-receptor protein tyrosine kinases (PTKs) that are crucial for cytokine receptor signaling in blood formation and immune responses. Mutations and translocations in the JAK genes leading to constitutively active JAK proteins are associated with a variety of hematopoietic malignancies, including the myeloproliferative disorders (JAK2), acute lymphoblastic leukemia (JAK2), acute myeloid leukemia (JAK2, JAK1), acute megakaryoblastic leukemia (JAK2, JAK3) and T-cell precursor acute lymphoblastic leukemia (JAK1). In contrast, loss-of-function mutations of JAK3 and TYK2 lead to immunodeficiency. The role of JAKs as therapeutic targets is starting to expand, as more insights into their structure and activation mechanisms become available.

Multiple kinases in the interferon-gamma response

Janus kinases (JAKs) and signal transducers and activators of transcription (STATs) are essential for responses to interferons (IFNs), most cytokines, and some growth factors. JAK/STAT signaling is not, however, sufficient for a full IFN-gamma response. Here, a convenient, robust, and quantitative flow cytometry-based kinome-wide siRNA screen has identified nine additional kinases as required for the IFN-gamma class II HLA response, seven for an antiviral response, and two for the cytopathic response to encephalomyocarditis virus (EMCV). As one example, inhibition of the IFN-gamma response by siRNA to ataxia telangiectasia-mutated (ATM) differentially affects a spectrum of IFN-gamma-stimulated mRNAs, with inhibitions being seen as early as 1 h after IFN-gamma stimulation. The implication of ATM, with its previously recognized function in chromatin decondensation, in the control of transcription early in the IFN-gamma response highlights both a role for ATM in cytokine responses and a possible correlation with the chromatin decondensation recently observed in response to IFN-gamma in mammalian cells. This work has, therefore, revealed the simplicity, power, and convenience of quantitative flow cytometry-based siRNA screens, a requirement for ATM and multiple additional kinases in the IFN-gamma response and a possible requirement for two of these kinases in the cytopathic response to EMCV.

JAK and MPL mutations in myeloid malignancies

The Janus family of non-receptor tyrosine kinases (JAK1, JAK2, JAK3 and tyrosine kinase 2) transduces signals downstream of type I and II cytokine receptors via signal transducers and activators of transcription (STATs). JAK3 is important in lymphoid and JAK2 in myeloid cell proliferation and differentiation. The thrombopoietin receptor MPL is one of several JAK2 cognate receptors and is essential for myelopoiesis in general and megakaryopoiesis in particular. Germline loss-of-function (LOF) JAK3 and MPL mutations cause severe combined immunodeficiency and congenital amegakaryocytic thrombocytopenia, respectively. Germline gain-of-function (GOF) MPL mutation (MPLS505N) causes familial thrombocytosis. Somatic JAK3 (e.g. JAK3A572V, JAK3V722I, JAK3P132T) and fusion JAK2 (e.g. ETV6-JAK2, PCM1-JAK2, BCR-JAK2) mutations have respectively been described in acute megakaryocytic leukemia and acute leukemia/chronic myeloid malignancies. However, current attention is focused on JAK2 (e.g. JAK2V617F, JAK2 exon 12 mutations) and MPL (e.g. MPLW515L/K/S, MPLS505N) mutations associated with myeloproliferative neoplasms (MPNs). A JAK2 mutation, primarily JAK2V617F, is invariably associated with polycythemia vera (PV). The latter mutation also occurs in the majority of patients with essential thrombocythemia (ET) or primary myelofibrosis (PMF). MPL mutational frequency in MPNs is substantially less (<10%). In general, despite a certain degree of genotype - phenotype correlations, the prognostic relevance of harbouring one of these mutations, or their allele burden when present, remains dubious. Regardless, based on the logical assumption that amplified JAK-STAT signalling is central to the pathogenesis of PV, ET and PMF, several anti-JAK2 tyrosine kinase inhibitors have been developed and are currently being tested in humans with these disorders.

Role of the JAK-STAT pathway in protection of hydrogen peroxide preconditioning against apoptosis induced by oxidative stress in PC12 cells

The aim of this study was to investigate the role of JAK-STAT pathway in the cytoprotection afforded by preconditioning with H(2)O(2). It was shown that (1) Preconditioning with 100 micromol/L H(2)O(2) can markedly protect PC12 cells against apoptosis and cytotoxicity induced by 300 micromol/L H(2)O(2); (2) The expression and tyrosine phosphorylation of JAK2, not JAK1 were rapidly increased at 5 min after H(2)O(2) preconditioning; (3) The expression of STAT1 and STAT3 were significantly increased at 15 min after H(2)O(2) preconditioning, and the pTyr-STAT1 and pTyr-STAT3 were markedly increased at 60 min after H(2)O(2) preconditioning; (4) Pretreatment with the JAK inhibitor AG-490 (10 micromol/L) 20 min before H(2)O(2) preconditioning blocked not only the activation of JAK2, STAT1 and STAT3, but also the cytoprotection of H(2)O(2) preconditioning against apoptosis and cytotoxicity induced by oxidative stress. These findings suggested that preconditioning with H(2)O(2) activated the JAK-STAT pathway that played an important role in the cytoprotection induced by H(2)O(2) preconditioning.

Stat1 required for interferon-inducible but not constitutive responsiveness to extracellular dsRNA

Distinct but partially overlapping signaling pathways mediate the response to extracellular vs. intracellular sources of dsRNA, by toll-like receptor 3 (TLR3) and retinoic acid-inducible gene-I/melanoma differentiated gene 5 (RIG-I/mda-5), respectively. Different cell types signal through these pathways to widely varying de grees. We previously observed that exposure to extracellular dsRNA, delivered by its addition to the culture medium, could induce the interferon (IFN)-stimulated gene 56 (ISG56) in human HT1080 fibrosarcoma cells, but not the HT1080-derived cell line, U3A, which lacks functional Stat1. In this study, we further investigated the nature of the dsRNA signaling defect in U3A cells. We show that a defect affecting basal TLR3 mRNA expression prevents U3A cells from responding to extracellular dsRNA. This defect does not impair dsRNA signaling in response to viral infection or transfected dsRNA. Although U3A cells are deficient in Stat1, we found that Stat1 was not required for basal TLR3 expression because other cell lines lacking Stat1 expressed TLR3. Moreover, restoration of Stat1 expression failed to restore TLR3 mRNA expression in U3A cells. However, treatment of Stat1-restored U3A cells with either IFN-beta or IFN-gamma induced TLR3 expression and restored responsiveness to extracellular dsRNA. Our results demonstrate that Stat1 is critical for IFN-induced, not basal, responsiveness to extracellular dsRNA.

NS1 proteins of avian influenza A viruses can act as antagonists of the human alpha/beta interferon response

Many viruses, including human influenza A virus, have developed strategies for counteracting the host type I interferon (IFN) response. We have explored whether avian influenza viruses were less capable of combating the type I IFN response in mammalian cells, as this might be a determinant of host range restriction. A panel of avian influenza viruses isolated between 1927 and 1997 was assembled. The selected viruses showed variation in their ability to activate the expression of a reporter gene under the control of the IFN-beta promoter and in the levels of IFN induced in mammalian cells. Surprisingly, the avian NS1 proteins expressed alone or in the genetic background of a human influenza virus controlled IFN-beta induction in a manner similar to the NS1 protein of human strains. There was no direct correlation between the IFN-beta induction and replication of avian influenza viruses in human A549 cells. Nevertheless, human cells deficient in the type I IFN system showed enhanced replication of the avian viruses studied, implying that the human type I IFN response limits avian influenza viruses and can contribute to host range restriction.

In the absence of IGF-1 signaling, IFN-gamma suppresses human malignant T-cell growth

Several approaches to target insulin-like growth factor-1 (IGF-1) signaling have resulted in the inhibition of the growth of a broad range of tumor cells. Malignant T cells are insensitive to the antiproliferative effects of the interferon-gamma (IFN-gamma)/signal transducer and activator of transcription 1 (STAT1) pathway because of the IGF-1-dependent internalization of the IFN-gammaR2 signaling chain. Here we show that human malignant T cells are also resistant to the growth inhibitory effect of both the IGF-1 receptor-specific inhibitor picropodophyllin (PPP) and retrovirus-mediated gene transfer of a dominant negative IGF-1 receptor. However, blockade of IGF-1 receptor perturbs IFN-gammaR2 internalization and induces its cell surface accumulation in malignant T cells. This allows the reinstatement of the IFN-gamma-induced STAT1 activation, a high expression of proapoptotic molecules, and the suppression of malignant T-cell growth both in vitro and in vivo in a severe combined immunodeficiency (SCID) mouse model. These data indicate that the inhibition of IGF-1 signaling combined with IFN-gamma administration could be a promising approach to suppress the growth of neoplastic T cells resistant to each treatment on its own.

Interferon: The pathways of discovery

This historical account covers 50 years of seminal research work on interferon done since its discovery in 1957. Topics related to molecular structure, production and action of interferons are considered from the viewpoint of how our insights have expanded and deepened within the context of evolving tools and general knowledge in cellular and molecular biology. Lines of thought that linked each discovery to the next are expounded.

The role of Janus kinases in haemopoiesis and haematological malignancy

The production of blood cells is regulated by a number of protein growth factors and cytokines that influence cell survival, proliferation and differentiation. Many of these molecules bind to cell surface receptors, which belong to a family of closely related cytokine receptors that lack intrinsic catalytic activity but are intimately associated with tyrosine kinases of the Janus kinase (JAK) family. Ligand binding induces the activation of JAKs, which sit at the apex of a signalling cascade in which a key role is played by members of the signal transducers and activators of transcription (STAT) group. Congenital deficiencies in JAK-STAT signalling are associated with immunodeficiency states and acquired activating mutations and translocations are involved in the pathophysiology of haematological malignancy. The latter findings have raised hopes that drugs that target aberrant JAK-STAT signalling may be useful for the treatment of human disease.

Preassembly and ligand-induced restructuring of the chains of the IFN-gamma receptor complex: the roles of Jak kinases, Stat1 and the receptor chains

We previously demonstrated using noninvasive technologies that the interferon-gamma (IFN-gamma) receptor complex is preassembled (1). In this report we determined how the receptor complex is preassembled and how the ligand-mediated conformational changes occur. The interaction of Stat1 with IFN-gammaR1 results in a conformational change localized to IFN-gammaR1. Jak1 but not Jak2 is required for the two chains of the IFN-gamma receptor complex (IFN-gammaR1 and IFN-gammaR2) to interact; however, the presence of both Jak1 and Jak2 is required to see any ligand-dependant conformational change. Two IFN-gammaR2 chains interact through species-specific determinants in their extracellular domains. Finally, these determinants also participate in the interaction of IFN-gammaR2 with IFN-gammaR1. These results agree with a detailed model of the IFN-gamma receptor that requires the receptor chains to be pre-associated constitutively for the receptor to be active.

Interleukins and STAT signaling

Metazoan cells secrete small proteins termed cytokines that execute a variety of biological functions essential for the survival of organisms. Binding of cytokines that belong to the hematopoietin- or interferon-family, to their cognate receptors on the surface of target cells, induces receptor aggregation, which in turn sequentially triggers tyrosine-phosphorylation-dependent activation of receptor-associated Janus-family tyrosine kinases (JAKs), receptors, and signal transducers and activators of transcription (STATs). Phosphorylated STATs form dimers that migrate to the nucleus, bind to cognate enhancer elements and activate transcription of target genes. Each cytokine activates a specific set of genes to execute its biological functions with a certain degree of redundancy. Cytokine signals are, in general, transient in nature. Therefore, under normal physiological conditions, initiation and attenuation of cytokine signals are tightly controlled via multiple cellular and molecular mechanisms. Aberrant activation of cytokine signaling pathways is, however, found under a variety of patho-physiological conditions including cancer and immune diseases.

Janus kinase 2 modulates the lipid-removing but not protein-stabilizing interactions of amphipathic helices with ABCA1

ABCA1 mediates the transport of cellular cholesterol and phospholipids to HDL apolipoproteins. Apolipoprotein A-I (apoA-I) interactions with ABCA1-expressing cells elicit several responses, including removing cellular lipids, stabilizing ABCA1 protein, and activating Janus kinase 2 (JAK2). Here, we used synthetic apolipoprotein-mimetic peptides to characterize the relationship between these responses. Peptides containing one amphipathic helix of L- or D-amino acids (2F, D-2F, or 4F) and a peptide containing two helices (37pA) all promoted ABCA1-dependent cholesterol efflux, competed for apoA-I binding to ABCA1-expressing cells, blocked covalent cross-linking of apoA-I to ABCA1, and inhibited ABCA1 degradation. 37pA was cross-linked to ABCA1, confirming the direct binding of amphipathic helices to ABCA1. 2F, 4F, 37pA, and D-37pA all stimulated JAK2 autophosphorylation. Inhibition of JAK2 greatly reduced peptide-mediated cholesterol efflux, peptide binding to ABCA1-expressing cells, and peptide cross-linking to ABCA1, indicating that these processes require an active JAK2. In contrast, apoA-I and peptides stabilized ABCA1 protein even in the absence of an active JAK2, implying that this process is independent of JAK2 and lipid efflux-promoting binding of amphipathic helices to ABCA1. These findings show that amphipathic helices coordinate the activity of ABCA1 by several distinct mechanisms that are likely to involve different cell surface binding sites.

Aldosterone induces angiotensin converting enzyme gene expression via a JAK2-dependent pathway in rat endothelial cells

Aldosterone is currently recognized as a risk hormone for cardiovascular disease. However, the cellular mechanism by which aldosterone acts on vasculature has not been well understood. In the present study, we investigated whether aldosterone affects angiotensin-converting enzyme (ACE) gene expression in rat endothelial cells. Cultured rat aortic endothelial cells (RAECs) from Sprague-Dawley rats were used in the study. ACE mRNA levels and its enzyme activities in RAECs were examined by real-time RT-PCR and enzyme assay using hippuryl-His-Leu as substrates, respectively. Aldosterone significantly increased steady-state ACE mRNA levels and its enzymatic activities. This effect was dose dependent and time dependent and abolished by mineralocorticoid receptor antagonist spironolactone or transcription inhibitor actinomycin D. Dexamethasone also increased steady-state ACE mRNA levels, whose effect was completely blocked by glucocorticoid receptor antagonist RU486, but not by spironolactone. By contrast, the aldosterone-induced ACE mRNA expression was only partially blocked by RU486. The stimulatory effect of aldosterone on ACE mRNA expression was completely blocked by a protein tyrosine kinase inhibitor (genistein) and JAK2 inhibitor (AG490), partially by Src kinase inhibitor (PP2) and epidermal growth factor receptor kinase inhibitor (AG1478), but not by platelet-derived growth factor receptor kinase inhibitor (AG1296). Transfection of dominant-negative JAK2 construct, but not wild-type construct, significantly blocked the aldosterone-induced ACE mRNA up-regulation. Furthermore, aldosterone induced phosphorylation of JAK2, whose effect was blocked by spironolactone and actinomycin D. In conclusion, the present study demonstrates for the first time that aldosterone induces ACE gene expression and its enzyme activity mainly via a mineralocorticoid receptor-mediated and JAK2-dependent pathway in rat endothelial cells. This may constitute a positive feedback loop for a local renin-angiotensin system, possibly involved in the development of aldosterone-induced endothelial dysfunction and vascular injury.

The Jak-STAT signaling pathway is required but not sufficient for the antiviral response of drosophila

The response of drosophila to bacterial and fungal infections involves two signaling pathways, Toll and Imd, which both activate members of the transcription factor NF-kappaB family. Here we have studied the global transcriptional response of flies to infection with drosophila C virus. Viral infection induced a set of genes distinct from those regulated by the Toll or Imd pathways and triggered a signal transducer and activator of transcription (STAT) DNA-binding activity. Genetic experiments showed that the Jak kinase Hopscotch was involved in the control of the viral load in infected flies and was required but not sufficient for the induction of some virus-regulated genes. Our results indicate that in addition to Toll and Imd, a third, evolutionary conserved innate immunity pathway functions in drosophila and counters viral infection.

Interleukin-6: from basic science to medicine--40 years in immunology

This essay summarizes my 40 years of research in immunology. As a young physician, I encountered a patient with Waldenström's macroglobulinemia, and this inspired me to study the structure of IgM. I began to ask how antibody responses are regulated. In the late 1960s, the essential role of T cells in antibody production had been reported. In search of molecules mediating T cell helper function, I discovered activities in the culture supernatant of T cells that induced proliferation and differentiation of B cells. This led to my life's work: studying one of those factors, interleukin-6 (IL-6). To my surprise, IL-6 turned out to play additional roles, including myeloma growth factor and hepatocyte-stimulating factor activities. More importantly, it was involved in a number of diseases, such as rheumatoid arthritis and Castleman's disease. I feel exceptionally fortunate that my work not only revealed the framework of cytokine signaling, including identification of the IL-6 receptor, gp130, NF-IL6, STAT3, and SOCS-1, but also led to the development of a new therapy for chronic inflammatory diseases.

Role of NF-kappa B in regulation of apoptosis of erythroid progenitor cells

Erythropoietin (EPO) and interferon-gamma (IFN-gamma) added to human erythroid progenitor cells purified from peripheral blood (erythroid colony-forming cells; ECFC) significantly reduces apoptosis as assessed by flow cytometry (FCM) using annexin V. To clarify the role of NF-kappaB in the regulation of the apoptosis of erythroid progenitor cells, cyclosporin A (CsA), which blocks dissociation of the NF-kappaB complex, was added to serum-free cultures of ECFC. CsA induced the apoptosis of ECFCs in the presence of EPO or IFN-gamma, but at different magnitudes. In the presence of a relatively low concentration of CsA (10 microm), apoptosis was induced only in cultures with EPO. The direct involvement of NF-kappaB was then assessed by Western blotting and confocal microscopy. In the presence of EPO, NF-kappaB was abundant both in the cytoplasm and in the nucleus, and nuclear expression was diminished after adding CsA. In contrast, NF-kappaB was undetectable in the nucleus in the presence of IFN-gamma. The effect of CsA on mitochondrial function was investigated by determining the DeltaPsim and reactive oxygen species production. CsA disturbed the transmembrane potential in the presence of either EPO or IFN-gamma, although the viability of the cells was maintained in the presence of IFN-gamma plus CsA. These results indicate that IFN-gamma reduced the apoptosis of erythroid progenitor cells through a unique signaling pathway that is independent of NF-kappaB translocation, and which is not mediated by modulating mitochondrial function, whereas EPO reduced apoptosis through NF-kappaB translocation to the nucleus.

Jak3, severe combined immunodeficiency, and a new class of immunosuppressive drugs

The recent elucidation of the multiple molecular mechanisms underlying severe combined immunodeficiency (SCID) is an impressive example of the power of molecular medicine. Analysis of patients and the concomitant generation of animal models mimicking these disorders have quickly provided great insights into the pathophysiology of these potentially devastating illnesses. In this review, we summarize the discoveries that led to the understanding of the role of cytokine receptors and a specific tyrosine kinase, Janus kinase 3 (Jak3), in the pathogenesis of SCID. We discuss how the identification of mutations of Jak3 in autosomal recessive SCID has facilitated the diagnosis of these disorders, offered new insights into the biology of this kinase, permitted new avenues for therapy, and provided the rationale for a generation of a new class of immunosuppressants.

Determination of the transphosphorylation sites of Jak2 kinase

Janus kinases are the key enzymes involved in the initial transmission of signals in response to type I and II cytokines. Activation of the signal begins with the transphosphorylation of Jak kinases. Substrates that give rise to downstream events are recruited to the receptor complex in part by interactions with phosphorylated tyrosines. The identity of many of the phosphotyrosines responsible for recruitment has been elucidated as being receptor-based tyrosines. The ability of Jaks to recruit substrates through their own phosphotyrosines has been demonstrated for tyrosines in the kinase activation loop. Recent studies demonstrate that other tyrosines have implications in regulatory roles of Jak kinase activity. In this study, baculovirus-produced Jak2 was utilized to demonstrate that transphosphorylation of Jak kinases occurs on multiple residues throughout the protein. We demonstrate that among the tyrosines phosphorylated, those in the kinase domain occur as expected, but many other sites are also phosphorylated. The tyrosines conserved in the Jak family are the object of this study, although many of them are phosphorylated, many are not. This result suggests that conservation of tyrosines is perhaps as important in maintaining structure of the Jak family. Additionally, non-Jak family conserved tyrosines are phosphorylated suggesting that the individual Jaks ability to phosphorylated specific tyrosines may influence signals emitting from activated Jaks.

A genetic screen to identify latent transforming growth factor beta activators

The mechanisms by which latent transforming growth factor beta (TGFbeta) is converted to the active cytokine are largely unknown. Here we present a genetic screen that combines retroviral mutagenesis and cDNA expression cloning to reveal proteins involved in the extracellular regulation of latent TGFbeta activation. The screen employs a cell line engineered to express green fluorescent protein (GFP) in response to TGFbeta. The cells produce their own latent TGFbeta. Therefore, after transduction with a retroviral cDNA library that contains an insert for an activator of latent TGFbeta, cells expressing the activator are GFP-bright. These cells are enriched by fluorescence-activated cell sorting and grown as individual clones. The isolated clones are cocultured with a second TGFbeta reporter cell line that produces luciferase in response to TGFbeta. Cells that have acquired the ability to activate latent TGFbeta induce luciferase expression in the absence but not in the presence of neutralizing antibodies to TGFbeta. The activator expressed by the positive clones can be identified by retrieval of the retrovirus cDNA insert.

An indispensable role for STAT1 in IL-27-induced T-bet expression but not proliferation of naive CD4+ T cells

IL-27 is a novel IL-12 family member that plays a role in the early regulation of Th1 initiation, induces proliferation of naive CD4+ T cells, and synergizes with IL-12 in IFN-gamma production. It has been recently reported that IL-27 induces T-bet and IL-12Rbeta2 expression through JAK1/STAT1 activation. In the present study, we further investigated the JAK/STAT signaling molecules activated by IL-27 and also the role of STAT1 in IL-27-mediated responses using STAT1-deficient mice. In addition to JAK1 and STAT1, IL-27-activated JAK2, tyrosine kinase-2, and STAT2, -3, and -5 in naive CD4+ T cells. The activation of STAT2 and STAT5, but not of STAT3, was greatly diminished in STAT1-deficient naive CD4+ T cells. Comparable proliferative response to IL-27 was observed between STAT1-deficient and wild-type naive CD4+ T cells. In contrast, IL-27 hardly induced T-bet and subsequent IL-12Rbeta2 expression, and synergistic IFN-gamma production by IL-27 and IL-12 was impaired in STAT1-deficient naive CD4+ T cells. Moreover, IL-27 augmented the expression of MHC class I on naive CD4+ T cells in a STAT1-dependent manner. These results suggest that IL-27 activates JAK1 and -2, tyrosine kinase-2, STAT1, -2, -3, and -5 in naive CD4+ T cells and that STAT1 plays an indispensable role in IL-27-induced T-bet and subsequent IL-12Rbeta2 expression and MHC class I expression as well but not proliferation, while STAT3 presumably plays an important role in IL-27-induced proliferation.

Induction of IgG2a class switching in B cells by IL-27

IL-27 is a novel IL-12 family member that plays a role in the early regulation of Th1 initiation. However, its role in B cells remains unexplored. We here show a role for IL-27 in the induction of T-bet expression and regulation of Ig class switching in B cells. Expression of WSX-1, one subunit of IL-27R, was detected at the mRNA level in primary mouse spleen B cells, and stimulation of these B cells by IL-27 rapidly activated STAT1. IL-27 then induced T-bet expression and IgG2a, but not IgG1, class switching in B cells activated with anti-CD40 or LPS. In contrast, IL-27 inhibited IgG1 class switching induced by IL-4 in activated B cells. Similar induction of STAT1 activation, T-bet expression and IgG2a class switching was observed in IFN-gamma-deficient B cells, but not in STAT1-deficient ones. The induction of IgG2a class switching was abolished in T-bet-deficient B cells activated with LPS. These results suggest that primary spleen B cells express functional IL-27R and that the stimulation of these B cells by IL-27 induces T-bet expression and IgG2a, but not IgG1, class switching in a STAT1-dependent but IFN-gamma-independent manner. The IL-27-induced IgG2a class switching is highly dependent on T-bet in response to T-independent stimuli such as LPS. Thus, IL-27 may be a novel attractive candidate as a therapeutic agent against diseases such as allergic disorders by not only regulating Th1 differentiation but also directly acting on B cells and inducing IgG2a class switching.

Generation and characterization of a constitutively active Stat3 protein

Stats are latent transcription factors involved in normal cellular signaling in response to cytokine or growth factor stimulation. Constitutive activation of Stats (primarily Stat3 and Stat5) has been implicated in growth dysregulation and oncogenesis. Furthermore, increased activation of Stats has been observed in several human tumors and tumor-derived cell lines. To assess the contribution of aberrant Stat activation in oncogenesis, we have created a chimeric molecule between Stat3beta and a portion of the Herpes simplex virus VP16 activation domain. The resulting protein, Stat3beta-VAD (VP16 activation domain), is tyrosine phosphorylated on Y705 and can bind DNA in the absence of upstream activation by c-Src or epidermal growth factor (EGF). Unlike Stat3alpha and Stat3beta, Stat3beta-VAD robustly activates transcription of several reporter genes without cytokine or growth factor stimulation. In addition, we show marked upregulation of the endogenous c-myc and c-fos genes upon inducible expression of Stat3beta-VAD in COS-7 cells. Our protein displays the constitutive transcriptional activation of Stat3alpha seen in human tumors and will be a valuable tool in screens for Stat3-regulated genes. In response to the established Stat3 involvement in human cancers, Stat3beta-VAD will also facilitate assessing the contribution of other cancer signaling cascades in the context of aberrant Stat3alpha activity in cancer development and progression.

Alternate interferon signaling pathways

More than a half a century ago, interferons (IFN) were identified as antiviral cytokines. Since that discovery, IFN have been in the forefront of basic and clinical cytokine research. The pleiotropic nature of these cytokines continues to engage a large number of investigators to define their actions further. IFN paved the way for discovery of Janus tyrosine kinase (JAK)-signal transducing activators of transcription (STAT) pathways. A number of important tumor suppressive pathways are controlled by IFN. Several infectious pathogens counteract IFN-induced signaling pathways. Recent studies indicate that IFN activate several new protein kinases, including the MAP kinase family, and downstream transcription factors. This review not only details the established IFN signaling paradigms but also provides insights into emerging alternate signaling pathways and mechanisms of pathogen-induced signaling interference.

Janus kinase 2 modulates the apolipoprotein interactions with ABCA1 required for removing cellular cholesterol

ATP-binding cassette transporter A1 (ABCA1) mediates transport of cellular cholesterol and phospholipids to high density lipoprotein (HDL) apolipoproteins, such as apoA-I. ABCA1 mutations can cause a severe HDL deficiency and atherosclerosis. Here we show that the protein-tyrosine kinase (TK) Janus kinase 2 (JAK2) modulates the apolipoprotein interactions with ABCA1 required for removing cellular lipids. The protein kinase A (PKA) inhibitor H89, the TK inhibitor genistein, and the JAK2 inhibitor AG490 suppressed apoA-I-mediated cholesterol and phospholipid efflux from ABCA1-expressing cells without altering the membrane ABCA1 content. Whereas PKA inhibition had no effect on apoA-I binding to cells or to ABCA1, TK and JAK2 inhibition greatly reduced these activities. Conversely, PKA but not JAK2 inhibition significantly reduced the intrinsic cholesterol translocase activity of ABCA1. Mutant cells lacking JAK2 had a severely impaired apoA-I-mediated cholesterol and phospholipid efflux and apoA-I binding despite normal ABCA1 protein levels and near normal cholesterol translocase activity. Thus, although PKA modulates ABCA1 lipid transport activity, JAK2 appears to selectively modulate apolipoprotein interactions with ABCA1. TK-mediated phosphorylation of ABCA1 was undetectable, implicating the involvement of another JAK2-targeted protein. Acute incubation of ABCA1-expressing cells with apoA-I had no effect on ABCA1 phosphorylation but stimulated JAK2 autophosphorylation. These results suggest that the interaction of apolipoproteins with ABCA1-expressing cells activates JAK2, which in turn activates a process that enhances apolipoprotein interactions with ABCA1 and lipid removal from cells.

Limitin, an interferon-like cytokine, transduces inhibitory signals on B-cell growth through activation of Tyk2, but not Stat1, followed by induction and nuclear translocation of Daxx

OBJECTIVE

Limitin, an interferon-like cytokine, suppresses B lymphopoiesis through ligation of the interferon-alpha/beta (IFN-alpha/beta) receptor. The aim of this study was to examine the intracellular signal transduction pathways activated by limitin.

MATERIALS AND METHODS

The effects of limitin on cell growth, the activation of Jak kinase and Stat proteins, and the induction of interferon regulatory factor-1 (IRF-1) and Daxx were examined using the mouse pre-B-cell line 18.81, wild-type, and Tyk2-deficient mouse bone marrow cells. In addition, the change of localization of the Daxx protein after limitin treatment in wild-type and Tyk2-deficient mice was examined.

RESULTS

Limitin phosphorylates Tyk2, Jak1, Stat1, and Stat2 and rapidly induces IRF-1 mRNA production. Phosphorylation of Stat1 by limitin is partially dependent on Tyk2. Suppression of B-cell growth by limitin, however, is severely impaired in the absence of Tyk2, whereas it is unaffected by the absence of Stat1. Limitin also induces the expression and nuclear translocation of Daxx, which is essential for IFN-alpha-induced inhibition of B-lymphocyte development. The absence of Tyk2 abrogates this induction of Daxx expression and nuclear translocation.

CONCLUSIONS

Limitin suppresses B-cell growth through activation of Tyk2, resulting in the up-regulation and nuclear translocation of Daxx. This limitin-mediated signaling pathway does not require Stat1.

Intracellular signal transduction of interferon on the suppression of haematopoietic progenitor cell growth

Interferon (IFN)-alpha and IFN-gamma suppress the growth of haematopoietic progenitor cells. IFN-alpha activates Janus kinase-1 (Jak1) and Tyrosine kinase-2 (Tyk2), followed by the phosphorylation of the signal transducers and activators of transcription, Stat1 and Stat2. IFN-gamma activates Jak1 and Jak2, followed by the activation of Stat1. Activated Stats bind the promoter regions of IFN-inducible genes. We evaluated the role of Tyk2 and Stat1 in the IFN-mediated inhibition of haematopoietic progenitor cell growth. While IFN-alpha (1000 U/ml) suppressed the number of granulocyte-macrophage colony-forming units (CFU-GM) or erythroid burst-forming units (BFU-E) from wild-type mouse bone marrow cells, this suppression was partially inhibited by a deficiency in Tyk2 and completely inhibited by a deficiency in Stat1. High levels of IFN-alpha (10,000 U/ml) suppressed the CFU-GM or BFU-E obtained from Stat1-deficient mice, but did not suppress this growth in cells from Tyk2-deficient mice. Stat1 was phosphorylated by IFN-alpha in Tyk2-deficient cells, although the level of phosphorylation was weaker than that observed in wild type mice. Thus, the inhibitory signal on haematopoietic progenitor cells mediated by IFN-alpha may be transduced by two signalling pathways, one regulated by Tyk2 and the other dependent on Stat1. IFN-gamma also suppressed the number of CFU-GM or BFU-E, and this pathway was mediated by IFN-gamma in a Stat1-dependent manner, independently of Tyk2.

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.

The JAK/STAT pathway is not sufficient to sustain the antiproliferative response in an interferon-resistant human melanoma cell line

The mechanism of resistance of malignant melanoma to treatment with interferon-alpha is unknown, and currently there is no reliable method of predicting response. Signalling via the JAK/STAT pathway is known to mediate many interferon-regulated events and has been implicated in mediating the antiproliferative response. The objective of this study was to determine whether defects in JAK/STAT signalling may be responsible for interferon resistance. The in vitro response to interferon was determined in a panel of established melanoma cell lines, and the components and functioning of the JAK/STAT pathway were examined in sensitive and resistant cell lines. Two melanoma cell lines, characterized as sensitive (MM418) and resistant (MeWo) to the antiproliferative effect of interferon, were both shown by Western blotting to possess all the protein components of the JAK/STAT pathway, and were shown to be capable of producing functional transcription factors using an electrophoretic mobility shift assay and a ribonuclease protection assay of known interferon-induced genes. In addition, both cell lines had intact antiviral and HLA upregulation responses. These data suggest that there is no defect in the JAK/STAT pathway per se in the MeWo cell line, and that the substantial resistance to interferon must be mediated through components either downstream or additional to this signalling pathway. Others have shown JAK/STAT defects to be responsible for interferon resistance in some melanoma cell lines. However, our results highlight the likely heterogeneity in the mechanisms leading to interferon resistance both in cell lines and tumours, and suggest that a clinical assay based on analysis of components of the JAK/STAT pathway may have only limited use as a predictor of interferon response.

Autoinhibition of Jak2 tyrosine kinase is dependent on specific regions in its pseudokinase domain

Jak tyrosine kinases have a unique domain structure containing a kinase domain (JH1) adjacent to a catalytically inactive pseudokinase domain (JH2). JH2 is crucial for inhibition of basal Jak activity, but the mechanism of this regulation has remained elusive. We show that JH2 negatively regulated Jak2 in bacterial cells, indicating that regulation is an intrinsic property of Jak2. JH2 suppressed basal Jak2 activity by lowering the V(max) of Jak2, whereas JH2 did not affect the K(m) of Jak2 for a peptide substrate. Three inhibitory regions (IR1-3) within JH2 were identified. IR3 (residues 758-807), at the C terminus of JH2, directly inhibited JH1, suggesting an inhibitory interaction between IR3 and JH1. Molecular modeling of JH2 showed that IR3 could form a stable alpha-helical fold, supporting that IR3 could independently inhibit JH1. IR2 (725-757) in the C-terminal lobe of JH2, and IR1 (619-670), extending from the N-terminal to the C-terminal lobe, enhanced IR3-mediated inhibition of JH1. Disruption of IR3 either by mutations or a small deletion increased basal Jak2 activity, but abolished interferon-gamma-inducible signaling. Together, the results provide evidence for autoinhibition of a Jak family kinase and identify JH2 regions important for autoregulation of Jak2.

Transport of exogenous growth factors and cytokines to the cytosol and to the nucleus

In recent years a number of growth factors, cytokines, protein hormones, and other proteins have been found in the nucleus after having been added externally to cells. This review evaluates the evidence that translocation takes place and discusses possible mechanisms. As a demonstration of the principle that extracellular proteins can penetrate cellular membranes and reach the cytosol, a brief overview of the penetration mechanism of protein toxins with intracellular sites of action is given. Then problems and pitfalls in attempts to demonstrate the presence of proteins in the cytosol and in the nucleus as opposed to intracellular vesicular compartments are discussed, and some new approaches to study this are described. A detailed overview of the evidence for translocation of fibroblast growth factor, HIV-Tat, interferon-gamma, and other proteins where there is evidence for intracellular action is given, and translocation mechanisms are discussed. It is concluded that although there are many pitfalls, the bulk of the experiments indicate that certain proteins are indeed able to enter the cytosol and nucleus. Possible roles of the internalized proteins are discussed.

The pseudokinase domain is required for suppression of basal activity of Jak2 and Jak3 tyrosine kinases and for cytokine-inducible activation of signal transduction

Janus (Jak) tyrosine kinases contain a tyrosine kinase (JH1) domain adjacent to a catalytically inactive pseudokinase domain (JH2). The JH2 domain has been implicated in regulation of Jak activity, but its function remains poorly understood. Here, we found that the JH2 domain negatively regulates the activity of Jak2 and Jak3. Deletion of JH2 resulted in increased tyrosine phosphorylation of the Jak2- and Jak3-JH2 deletion mutants as well as of coexpressed STAT5. In cytokine receptor signaling, the deletion of the Jak2- and Jak3-JH2 domains resulted in interferon-gamma and interleukin-2-independent STAT activation, respectively. However, cytokine stimulations did not further induce the JH2 deletion mutant-mediated STAT activation. The deletion of the Jak2 JH2 domain also abolished interferon-gamma-inducible kinase activation, although it did not affect the reciprocal Jak1-Jak2 interaction in 293T cells. Chimeric constructs, where the JH2 domains were swapped between Jak2 and Jak3, retained low basal activity and cytokine inducible signaling, indicating functional conservation between the two JH2 domains. However, the basal activity of Jak2 was significantly lower than that of Jak3, suggesting differences in the regulation of Jak2 and Jak3 activity. In conclusion, we found that the JH2 domain has a conserved function in Jak2 and Jak3. The JH2 domain is required for two distinct functions in cytokine signaling: (i) inhibition of the basal activity of Jak2 and Jak3, and (ii) cytokine-inducible activation of signaling. The Jak-JH2 deletion mutants are catalytically active, activate STAT5, and interact with another Jak kinase, but the JH2 domain is required to connect these signaling events to receptor activation. Thus, we propose that the JH2 domain contributes to both the uninduced and ligand-induced Jak-receptor complex, where it acts as a cytokine-inducible switch to regulate signal transduction.

Dual mechanism of signal transducer and activator of transcription 5 activation by the insulin receptor

Insulin stimulates signal transducer and activator of transcription 5 (Stat5) activation in insulin receptor (IR)-overexpressing cell lines and in insulin target tissues of mice. Stat5b and insulin receptor substrate 1 (IRS-1) interact with the same autophosphorylation site in the IR [phosphotyrosine (pY) 972] in yeast two-hybrid assays, and the IR phosphorylates Stat5b in vitro. These data suggest that Stat5 proteins might be recruited to, and phosphorylated by, the activated IR in vivo. Nevertheless, insulin activates Janus kinases (JAKs) in IR-overexpressing cell lines and in insulin target tissues. To determine whether Stat5 proteins must be recruited to the pY972LSA motif in the IR for insulin-stimulated activation in mammalian cells, we generated and tested a series of IR mutants. The L973R/A975D mutation abolishes the ability of the IR to induce Stat5 activation, whereas IRS-1 phosphorylation is unaffected. In contrast, the N969A/P970A mutation in the IR has no effect on Stat5 activation but significantly reduces IRS-1 phosphorylation. In coimmunoprecipitation assays, insulin-stimulated Stat5 activation correlates with Stat5 recruitment to the IR. We also find that insulin stimulates tyrosine phosphorylation of JAKs that are constitutively associated with the IR. Expression of dominant-negative (DN) JAKs, the JAK inhibitor suppressor of cytokine signaling 1, or pretreatment with the JAK inhibitor, AG490, reduces, but does not eliminate, insulin-induced Stat5 activation. Expression of the appropriate pair of DN JAKs in each of the singly JAK-deficient cell lines further establishes a component of insulin-stimulated Stat5 activation that is JAK independent. This likely represents phosphorylation of Stat5 proteins by the IR, as we find that IR kinase domain phosphorylates Stat5b in vitro on Y699 as efficiently as JAK2. Increasing the concentration of Stat5 proteins in cells favors the direct phosphorylation of Stat5 by the IR kinase where the DN-JAK inhibition of insulin-stimulated Stat5 activation becomes insignificant. At physiological levels of Stat5 however, we propose that JAKs and the IR both contribute to the insulin-induced phosphorylation of Stat5.

Cutting edge: tyk2 is required for the induction and nuclear translocation of Daxx which regulates IFN-alpha-induced suppression of B lymphocyte formation

IFN-alpha inhibits B lymphocyte development, and the nuclear protein Daxx has been reported to be essential for this biological activity. We show in this study that IFN-alpha inhibits the clonal proliferation of B lymphocyte progenitors in response to IL-7 in wild-type, but not in tyk2-deficient, mice. In addition, the IFN-alpha-induced up-regulation and nuclear translocation of Daxx are completely abrogated in the absence of tyk2. Therefore, tyk2 is directly involved in IFN-alpha signaling for the induction and translocation of Daxx, which may result in B lymphocyte growth arrest and/or apoptosis.