Janus kinases in interleukin-2-mediated signaling: JAK1 and JAK3 are differentially regulated by tyrosine phosphorylation

IL-7 in autoimmune diseases: mechanisms and therapeutic potential

Interleukin-7 (IL-7) is a pleiotropic cytokine that plays a crucial role in the development, homeostasis, and function of the immune system. Growing evidence has demonstrated that IL-7 is involved in the pathogenesis of various autoimmune diseases including rheumatoid arthritis (RA), systemic lupus erythematosus (SLE), type 1 diabetes (T1D) and multiple sclerosis (MS). This review aims to summarize the current understanding of the role of IL-7 in autoimmune diseases, focusing on its mechanisms of action, implications for disease progression, and potential therapeutic applications. Produced by stromal cells, IL-7 binds to IL-7 receptor (IL-7R) on diverse immune cells. It is crucial for T cell development, survival, and proliferation. In autoimmune diseases, it activates and expands autoreactive T cells and influences B cell function, potentially leading to autoantibody production. The review further delves into the role of IL-7 in different autoimmune diseases. In RA, elevated IL-7/IL-7R promotes memory T cell survival, cytokine production, and influences B cells and monocytes to contribute to inflammation and joint damage. In SLE, elevated soluble form of IL-7R is associated with disease activity, promoting the survival of autoreactive T cells and enhancing the production of pro-inflammatory cytokines. In MS, genetic variations in the IL-7R gene are linked to disease susceptibility, and IL-7 impacts the survival and differentiation of T cell subsets involved in multiple sclerosis pathogenesis. For T1D, IL-7 affects the function of immune cells that attack pancreatic β cells. Given its central role in autoimmune processes, targeting the IL-7/IL-7R axis holds great therapeutic potential. By modulating IL-7 signaling, it may be possible to restore immune tolerance, reduce the activation of autoreactive immune cells, and alleviate disease symptoms. Understanding the complex mechanisms of IL-7 in autoimmune diseases is essential for the development of effective and targeted therapies.

Activity Regulation and Conformation Response of Janus Kinase 3 Mediated by Phosphorylation: Exploration from Correlation Network Analysis and Markov Model

The activity of the enzyme JAK3 is modulated by tyrosine phosphorylation, yet the underlying molecular details remain not fully understood. In this study, we employed a GaMD trajectory-based Markov model and correlation network analysis (CNA) to investigate the impact of single phosphorylation (SP) at Y980 (pY980) and double phosphorylation (DP) at Y980/Y981 (pY980/pY981) on the conformational dynamics of JAK3 bound by inhibitors IZA and MI1. The Markov model analysis indicated that both SP and DP result in fewer conformational states and significantly influence the conformational dynamics of the P-loop, αC-helix, and loop1-loop3, while maintaining the hinge region's high rigidity. The CNA findings revealed that phosphorylation alters the communication network among different structural regions of JAK3, providing a rational explanation for how phosphorylation affects the conformational dynamics of the distant P-loop and loop1-loop3. Moreover, the conformational changes mediated by SP and DP further affect the interactions between the inhibitors and the hot spots (L828, V836, E903, Y904, L905, and L956) of JAK3. This work offers valuable theoretical insights into the molecular mechanisms that regulate JAK3 activity.

IL-7 Immunotherapies: Current Applications and Engineering Opportunities

BACKGROUND

IL-7 is a cytokine that plays a critical role in the development and proliferation of many different immune cells. IL-7 is notably important for the proper development and activity of T cells and B cells. Additionally, the cytokine plays a role in the function of natural killer cells and dendritic cells. Because of this innate biological activity, IL-7 has gained traction as a potential immunotherapy for multiple applications.

METHODS

We conducted a comprehensive literature review to explore the physiological role of IL-7 and current applications harnessing the biology of IL-7 as a therapeutic. We also investigated the ways in which IL-7 is being engineered to enhance its therapeutic potential.

RESULTS

Notably, IL-7 has demonstrated efficacy in adoptive cell therapy models and as a vaccine adjuvant. The cytokine has also been used as a treatment for sepsis and other chronic infections. To further enhance its therapeutic efficacy, IL-7 has been engineered by fusing the cytokine to antibody fragments or other bioactive or targeting molecules. These engineered IL-7 therapeutics seek to improve the cytokine's pharmacokinetic and immunological properties and reduce off-target effects.

CONCLUSION

IL-7 immunotherapies largely remain at the preclinical stage, but there is growing interest in IL-7's many therapeutic applications and increasing opportunities to further engineer the molecule for future clinical translation.

Promiscuous Janus kinase binding to cytokine receptors modulates signaling efficiencies and contributes to cytokine pleiotropy

Janus kinases (JAKs) bind to class I and II cytokine receptors, activating signaling and regulating gene transcription through signal transducer and activator of transcription (STAT) proteins. Type I interferons (IFNs) require the JAK members TYK2 and JAK1, which bind to the receptor subunits IFNAR1 and IFNAR2, respectively. We investigated the role of JAKs in regulating IFNAR signaling activity. Synthetic IFNARs in which the extracellular domains of IFNAR1 and IFNAR2 are replaced with nanobodies had near-native type I IFN signaling, whereas the homomeric variant of IFNAR2 initiated much weaker signaling, despite harboring docking sites for JAKs and STATs. Cells with JAK1 and TYK2 knockout (KO) showed residual signaling, suggesting partial complementation by the remaining JAKs, particularly when they were overexpressed. Live-cell micropatterning experiments confirmed the promiscuous binding of JAK1, JAK2, and TYK2 to IFNAR1 and IFNAR2, and their recruitment correlated with their relative cellular abundances. However, each JAK had a different efficacy in inducing cross-phosphorylation and downstream signaling. JAK binding was also promiscuous for other cytokine receptors, including IFN-L1, IL-10Rβ, TPOR, and GHR, but not for EPOR, which activated different downstream signaling pathways. These findings suggest that competitive binding of JAKs to cytokine receptors together with the varying absolute and relative abundances of the JAKs in different cell types can account for the cell type-dependent signaling pleiotropy of cytokine receptors.

An improved SNAP-ADAR tool enables efficient RNA base editing to interfere with post-translational protein modification

RNA base editing relies on the introduction of adenosine-to-inosine changes into target RNAs in a highly programmable manner in order to repair disease-causing mutations. Here, we propose that RNA base editing could be broadly applied to perturb protein function by removal of regulatory phosphorylation and acetylation sites. We demonstrate the feasibility on more than 70 sites in various signaling proteins and identify key determinants for high editing efficiency and potent down-stream effects. For the JAK/STAT pathway, we demonstrate both, negative and positive regulation. To achieve high editing efficiency over a broad codon scope, we applied an improved version of the SNAP-ADAR tool. The transient nature of RNA base editing enables the comparably fast (hours to days), dose-dependent (thus partial) and reversible manipulation of regulatory sites, which is a key advantage over DNA (base) editing approaches. In summary, PTM interference might become a valuable field of application of RNA base editing.

Palmitoylation-dependent control of JAK1 kinase signaling governs responses to neuropoietic cytokines and survival in DRG neurons

Janus Kinase-1 (JAK1) plays key roles during neurodevelopment and following neuronal injury, while activatory JAK1 mutations are linked to leukemia. In mice, Jak1 genetic deletion results in perinatal lethality, suggesting non-redundant roles and/or regulation of JAK1 for which other JAKs cannot compensate. Proteomic studies reveal that JAK1 is more likely palmitoylated compared to other JAKs, implicating palmitoylation as a possible JAK1-specific regulatory mechanism. However, the importance of palmitoylation for JAK1 signaling has not been addressed. Here, we report that JAK1 is palmitoylated in transfected HEK293T cells and endogenously in cultured Dorsal Root Ganglion (DRG) neurons. We further use comprehensive screening in transfected non-neuronal cells and shRNA-mediated knockdown in DRG neurons to identify the related enzymes ZDHHC3 and ZDHHC7 as dominant protein acyltransferases (PATs) for JAK1. Surprisingly, we found palmitoylation minimally affects JAK1 localization in neurons, but is critical for JAK1's kinase activity in cells and even in vitro. We propose this requirement is likely because palmitoylation facilitates transphosphorylation of key sites in JAK1's activation loop, a possibility consistent with structural models of JAK1. Importantly, we demonstrate a leukemia-associated JAK1 mutation overrides the palmitoylation-dependence of JAK1 activity, potentially explaining why this mutation is oncogenic. Finally, we show that JAK1 palmitoylation is important for neuropoietic cytokine-dependent signaling and neuronal survival and that combined Zdhhc3/7 loss phenocopies loss of palmitoyl-JAK1. These findings provide new insights into the control of JAK signaling in both physiological and pathological contexts.

IL-6 prevents Th2 cell polarization by promoting SOCS3-dependent suppression of IL-2 signaling

Defective interleukin-6 (IL-6) signaling has been associated with Th2 bias and elevated IgE levels. However, the underlying mechanism by which IL-6 prevents the development of Th2-driven diseases remains unknown. Using a model of house dust mite (HDM)-induced Th2 cell differentiation and allergic airway inflammation, we showed that IL-6 signaling in allergen-specific T cells was required to prevent Th2 cell differentiation and the subsequent IgE response and allergic inflammation. Th2 cell lineage commitment required strong sustained IL-2 signaling. We found that IL-6 turned off IL-2 signaling during early T-cell activation and thus inhibited Th2 priming. Mechanistically, IL-6-driven inhibition of IL-2 signaling in responding T cells was mediated by upregulation of Suppressor Of Cytokine Signaling 3 (SOCS3). This mechanism could be mimicked by pharmacological Janus Kinase-1 (JAK1) inhibition. Collectively, our results identify an unrecognized mechanism that prevents the development of unwanted Th2 cell responses and associated diseases and outline potential preventive interventions.

The role of upadacitinib in the treatment of ulcerative colitis

The JAK signaling pathway plays a major role in the immunopathology of autoimmune diseases, including inflammatory bowel disease. JAK enzymes provide novel targets for rapidly effective inflammatory bowel disease therapy, particularly in ulcerative colitis. Upadacitinib is a targeted JAK1 inhibitor. In multiple phase III clinical trials, upadacitinib has demonstrated significant improvement in clinical and endoscopic outcomes and quality of life for patients with moderate-to-severe ulcerative colitis. In this drug evaluation we describe the role of the JAK signaling pathway in ulcerative colitis, the mechanism of action of upadacitinib and the current clinical evidence for its use in ulcerative colitis; we also review its safety and tolerability, including for special populations.

Dynamic Roles for IL-2-STAT5 Signaling in Effector and Regulatory CD4+ T Cell Populations

CD4⁺ Th cells are responsible for orchestrating diverse, pathogen-specific immune responses through their differentiation into a number of subsets, including T_H1, T_H2, T_H9, T follicular helper, T follicular regulatory, and regulatory T cells. The differentiation of each subset is guided by distinct regulatory requirements, including those derived from extracellular cytokine signals. IL-2 has emerged as a critical immunomodulatory cytokine that both positively and negatively affects the differentiation of individual Th cell subsets. IL-2 signals are propagated, in part, via activation of STAT5, which functions as a key regulator of CD4⁺ T cell gene programs. In this review, we discuss current understanding of the mechanisms that allow IL-2-STAT5 signaling to exert divergent effects across CD4⁺ T cell subsets and highlight specific roles for this pathway in the regulation of individual Th cell differentiation programs.

Transcription Factors Associated With IL-15 Cytokine Signaling During NK Cell Development

Natural killer (NK) cells are lymphocytes primarily involved in innate immunity and possess important functional properties in anti-viral and anti-tumor responses; thus, these cells have broad potential for clinical utilization. NK cells originate from hematopoietic stem cells (HSCs) through the following two independent and continuous processes: early commitment from HSCs to IL-15-responsive NK cell progenitors (NKPs) and subsequent differentiation into mature NK cells in response to IL-15. IL-15 is the most important cytokine for NK cell development, is produced by both hematopoietic and nonhematopoietic cells, and functions through a distinct delivery process termed transpresentation. Upon being transpresented to NK cells, IL-15 contributes to NK cell development via the activation of several downstream signaling pathways, including the Ras-MEK-MAPK, JAK-STAT5, and PI3K-ATK-mTOR pathways. Nonetheless, the exact role of IL-15 in NK cell development has not been discussed in a consecutive and comprehensive manner. Here, we review current knowledge about the indispensable role of IL-15 in NK cell development and address which cells produce IL-15 to support NK cell development and when IL-15 exerts its function during multiple developmental stages. Specifically, we highlight how IL-15 supports NK cell development by elucidating the distinct transpresentation of IL-15 to NK cells and revealing the downstream target of IL-15 signaling during NK cell development.

Whole Genome Sequencing of Spontaneously Occurring Rat Natural Killer Large Granular Lymphocyte Leukemia Identifies JAK1 Somatic Activating Mutation

Large granular lymphocyte (LGL) leukemia arises spontaneously in elderly Fischer (F344) rats. This rodent model has been shown to emulate many aspects of the natural killer (NK) variant of human LGL leukemia. Previous transplantation of leukemic material into young F344 rats resulted in several strains of rat NK (RNK) primary leukemic cells. One strain, RNK-16, was adapted into the RNK-16 cell line and established as an aggressive NK-LGL leukemia model. Whole genome sequencing of the RNK-16 cell line identified 255,838 locations where the RNK16 had an alternate allele that was different from F334, including a mutation in Jak1. Functional studies showed Jak1 Y1034C to be a somatic activating mutation that mediated increased STAT signaling, as assessed by phosphoprotein levels. Sanger sequencing of Jak1 in RNK-1, -3, -7, and -16 found only RNK-16 to harbor the Y1034C Jak1 mutation. In vivo studies revealed that rats engrafted with RNK-16 primary material developed leukemia more rapidly than those engrafted with RNK-1, -3, and -7. Additionally, ex vivo RNK-16 spleen cells from leukemic rats exhibited increased STAT1, STAT3, and STAT5 phosphorylation compared to other RNK strains. Therefore, we report and characterize a novel gain-of-function Jak1 mutation in a spontaneous LGL leukemia model that results in increased downstream STAT signaling.

TPD7 inhibits the growth of cutaneous T cell lymphoma H9 cell through regulating IL-2R signalling pathway

IL‐2R pathway is a key regulator in the development of immune cells and has emerged as a promising drug target in cancer treatment, but there is a scarcity of related inhibitors. TPD7 is a novel biphenyl urea taspine derivate, which has been shown anti‐cancer effect. Here, we demonstrated the anti‐cancer activity of TPD7 in cutaneous T cell lymphoma and investigated the underlying mechanism of TPD7 through IL‐2R signalling. The inhibitory effect of TPD7 on cell viability exhibited a strong correlation with the expression level of IL‐2R, and cutaneous T cell lymphoma H9 and HUT78 cells were most sensitive to TPD7. TPD7 was nicely bound to IL‐2R and down‐regulated the mRNA and protein levels of IL‐2R. Furthermore, TPD7 suppressed the downstream cascades of IL‐2R including JAK/STAT, PI3K/AKT/mTOR and PLCγ/Raf/MAPK signalling, resulting in Bcl‐2 mitochondrial apoptosis pathway and cell cycle proteins CDK/Cyclins regulation. And, these were verified by flow cytometry analysis that TPD7 facilitated cell apoptosis in H9 cells via mitochondrial pathway and impeded cell cycle progression at G2/M phase. TPD7 is a novel anti‐cancer agent and may be a potential candidate for cutaneous T cell lymphoma treatment by regulating IL‐2R signalling pathway.

Molecular cloning and functional characterization of duck Janus kinase 1

Janus kinase 1 (JAK1) is a member of JAK family of non-receptor protein tyrosine kinases that plays critical roles in transducing cytokine signals via JAK-signal transducer and activator of transcription (STAT) signaling pathway. The importance of JAK1 in innate immunity has been well-studied in mammals and fish, yet in avian remains largely unknown. Here, we cloned the full-length of the duck JAK1 (duJAK1) gene for the first time. DuJAK1 encoded a protein of 1152 amino acids and possessed high amino acid identity with goose and budgerigar JAK1s. The duJAK1 was expressed in all detected tissues, especially high in the thymus and bursa of Fabricius. Overexpression of duJAK1 significantly activated ISRE promoter activity and induced duck viperin, 2', 5'-OAS, MX, PKR and ZAP expression. Knockdown of duJAK1 by small interfering RNA significantly inhibited duck Tembusu virus (DTMUV)-, duck Enteritis virus (DEV)-, poly (I:C)-, poly (dA:dT)- or Sendai virus (SeV)-induced ISRE promoter activation. Furthermore, duJAK1 exhibited antiviral activity against DTMUV infection. These results will help us understand the function of JAK family proteins in duck antiviral immunity.

Hyperactivation of Oncogenic JAK3 Mutants Depend on ATP Binding to the Pseudokinase Domain

Janus kinase 3 (JAK3) tyrosine kinase has a central role in the control of lymphopoiesis, and mutations in JAK3 can lead to either severe combined immunodeficiency or leukemia and lymphomas. JAK3 associates with the common gamma chain (γc) receptor and functions in a heteromeric signaling pair with JAK1. In IL-2 signaling JAK1 is the effector kinase for STAT5 phosphorylation but the precise molecular regulatory mechanisms of JAK1 and JAK3 and their individual domains are not known. The pseudokinase domain (JAK homology 2, JH2) of JAK3 is of particular interest as approximately half of clinical JAK3 mutations cluster into it.

In this study, we investigated the role of JH2s of JAK1 and JAK3 in IL-2R signaling and show that STAT5 activation requires both JH1 and JH2 of JAK1, while both JH1 and JH2 in JAK3 are specifically required for the cytokine-induction of cellular signaling. Characterization of recombinant JAK3 JH2 in thermal shift assay shows an unstable protein domain, which is strongly stabilized by ATP binding. Unexpectedly, nucleotide binding to JAK3 JH2 was found to be cation-independent. JAK3 JH2 showed higher nucleotide binding affinity in MANT-ATP and fluorescent polarization competition assays compared to the other JAK JH2s. Analysis of the functional role of ATP binding in JAK3 JH2 in cells and in zebrafish showed that disruption of ATP binding suppresses ligand-independent activation of clinical JAK3 gain-of-function mutations residing in either JH2 or JH1 but does not inhibit constitutive activation of oncogenic JAK1. ATP-binding site mutations in JAK3 JH2 do not, however, abrogate normal IL-2 signaling making them distinct from JH2 deletion or kinase-deficient JAK3. These findings underline the importance of JAK3 JH2 for cellular signaling in both ligand-dependent and in gain-of-function mutation-induced activation. Furthermore, they identify the JH2 ATP-binding site as a key regulatory region for oncogenic JAK3 signaling, and thus a potential target for therapeutic modulation.

Replacement of mycophenolate mofetil with a JAK inhibitor, AS2553627, in combination with low-dose tacrolimus, for renal allograft rejection in non-human primates

In renal transplant patients, using mycophenolate mofetil (MMF) with calcineurin inhibitors (CNIs; cyclosporine and tacrolimus [TAC]) has led to a significant improvement in graft survival. However, reducing or withholding MMF due to its gastrointestinal adverse events increases rejection risk. CNI-sparing strategies are important to avoid CNI-related nephrotoxicity in clinical settings. Here, we investigated AS2553627, a JAK inhibitor replacing MMF in combination with a sub-therapeutic dose of TAC to treat allograft rejection in a monkey model. AS2553627 inhibited proliferation of IL-2 stimulated T cells with little species difference between monkeys and humans. In MMF monotherapy, oral administration of 20 or 40 mg/kg/day prolonged graft survival with median survival times (MSTs) of 16.5 days and 33 days, respectively, whereas untreated animals showed MST of 6 days. In MMF/TAC (1 mg/kg/day, p.o.) combination therapy, pharmacokinetic analysis indicated that MMF 20 mg/kg/day achieved the clinical target AUC_0-24h and prolonged renal allograft survival, with MST of 24 days. Oral administration of AS2553627 0.24 mg/kg/day in combination with TAC significantly prolonged renal allograft survival to MST of >90 days with low plasma creatinine levels. Histopathological analysis revealed that acute T cell-mediated rejection events such as vasculitis and interstitial mononuclear cell infiltration were significantly inhibited in AS2553627/TAC-treated allografts compared with MMF/TAC-treated allografts. All AS2553627/TAC-treated monkeys surviving >90 days exhibited less interstitial fibrosis/tubular atrophy than monkeys in the MMF/TAC group. These results suggest that AS2553627 replacing MMF is an attractive CNI-sparing strategy to prevent renal allograft rejection.

Insights into kinetic mechanism of Janus kinase 3 and its inhibition by tofacitinib

JAK3 kinase plays a critical role in several cytokine signaling pathways involved in immune cell development and function. The studies presented in this report were undertaken to elucidate the kinetic mechanism of the JAK3 kinase domain, investigate the role of activation loop phosphorylation in regulating its catalytic activity, and examine its inhibition by the anti-rheumatoid arthritis drug, tofacitinib. Phosphorylation of two Tyr residues in JAK3's activation loop has been reported to impact its kinase activity. The recombinant JAK3 kinase domain used in our studies was heterogeneous in its activation loop phosphorylation, with the non-phosphorylated protein being the dominant species. Kinetic analysis revealed similar kinetic parameters for the heterogeneously phosphorylated JAK3, JAK3 mono-phosphorylated on Tyr 980, and the activation loop mutant YY980/981FF. Bisubstrate and product inhibition kinetic results were consistent with both sequential random and sequential ordered kinetic mechanisms. Solvent viscosometric experiments showed perturbation of k_cat, suggesting the phosphoryl transfer step is not likely rate limiting. This was supported by results from quench-flow experiments, where a rapid burst of product formation was observed. Kinetic analysis of JAK3 inhibition by tofacitinib indicated inhibition is time dependent, characterized by on- and off-rate constants of 1.4 ± 0.1 μM^-1s^-1 and 0.0016 ± 0.0005 s^-1, respectively.

Differential regulation of Janus kinase 3 (JAK3) in bovine preovulatory follicles and identification of JAK3 interacting proteins in granulosa cells

Background

Janus kinase 3 (JAK3) is a member of the membrane-associated non-receptor tyrosine kinase protein family and is considered predominantly expressed in hematopoietic cells. We previously identified JAK3 as a differentially expressed gene in granulosa cells (GC) of bovine preovulatory follicles. The present study aimed to further investigate JAK3 regulation, to identify protein binding partners and better understand its mode of action in bovine reproductive cells.

Results

GC were obtained from small follicles (SF), dominant follicles at day 5 of the estrous cycle (DF), and ovulatory follicles, 24 h following hCG injection (OF). RT-PCR analyses showed greatest expression of JAK3 in GC of DF, while JAK3 expression was downregulated in OF (P < 0.0001). In addition, there was a 5- and 20-fold reduction of JAK3 steady-state mRNA levels in follicular walls, respectively at 12 and 24 hours post-hCG as compared to 0 h (P < 0.05). Similarly, JAK3 expression was downregulated by the endogenous LH surge. These results were confirmed in western blot analysis showing weakest JAK3 protein amounts in OF as compared to DF. Yeast two-hybrid screening of a DF-cDNA library resulted in the identification of JAK3 partners in GC that were confirmed by co-immunoprecipitation and included leptin receptor overlapping transcript-like 1 (LEPROTL1), inhibin beta A (INHBA) and cyclin-dependent kinase inhibitor 1B (CDKN1B). In functional studies using bovine endometrial cells, JAK3 increased phosphorylation of STAT3 and cell viability, while the addition of JANEX-1 inhibited JAK3 actions.

Conclusion

These results support a physiologically relevant role of JAK3 in follicular development and provide insights into the mode of action and function of JAK3 in reproductive tissues.

Electronic supplementary material

The online version of this article (doi:10.1186/s13048-016-0280-5) contains supplementary material, which is available to authorized users.

Interleukin-4 receptor signaling and its binding mechanism: A therapeutic insight from inhibitors tool box

Studies on Interlukin-4 (IL-4) disclosed great deal of information about its various physiological and pathological roles. All these roles depend upon its interaction and signaling through either type-I (IL-4Rα/common γ-chain) or type-II (IL-4Rα/IL-13Rα) receptors. Another cytokine, IL-13, shares some of the functions of IL-4, because both cytokines use a common receptor subunit, IL-4Rα. Here in this review, we discuss the structural details of IL-4 and IL-4Rα subunit and the structural similarities between IL-4 and IL-13. We also describe detailed chemistry of type-I and type-II receptor complexes and their signaling pathways. Furthermore, we elaborate the strength of type-II hetero dimer signals in response to IL-4 and IL-13. These cytokines are prime players in pathogenesis of allergic asthma, allergic hypersensitivity, different cancers, and HIV infection. Recent advances in the structural and binding chemistry of these cytokines various types of inhibitors were designed to block the interaction of IL-4 and IL-13 with their receptor, including several IL-4 mutant analogs and IL-4 antagonistic antibodies. Moreover, different targeted immunotoxins, which is a fusion of cytokine protein with a toxin or suicidal gene, are the new class of inhibitors to prevent cancer progression. In addition few small molecular inhibitors such as flavonoids have also been developed which are capable of binding with high affinity to IL-4Rα and, therefore, can be very effective in blocking IL-4-mediated responses.

Structural and Functional Characterization of the JH2 Pseudokinase Domain of JAK Family Tyrosine Kinase 2 (TYK2)

JAK (Janus family of cytoplasmic tyrosine kinases) family tyrosine kinase 2 (TYK2) participates in signaling through cytokine receptors involved in immune responses and inflammation. JAKs are characterized by dual kinase domain: a tyrosine kinase domain (JH1) that is preceded by a pseudokinase domain (JH2). The majority of disease-associated mutations in JAKs map to JH2, demonstrating its central regulatory function. JH2s were considered catalytically inactive, but JAK2 JH2 was found to have low autoregulatory catalytic activity. Whether the other JAK JH2s share ATP binding and enzymatic activity has been unclear. Here we report the crystal structure of TYK2 JH2 in complex with adenosine 5'-O-(thiotriphosphate) (ATP-γS) and characterize its nucleotide binding by biochemical and biophysical methods. TYK2 JH2 did not show phosphotransfer activity, but it binds ATP and the nucleotide binding stabilizes the protein without inducing major conformational changes. Mutation of the JH2 ATP-binding pocket increased basal TYK2 phosphorylation and downstream signaling. The overall structural characteristics of TYK2 JH2 resemble JAK2 JH2, but distinct stabilizing molecular interactions around helix αAL in the activation loop provide a structural basis for differences in substrate access and catalytic activities among JAK family JH2s. The structural and biochemical data suggest that ATP binding is functionally important for both TYK2 and JAK2 JH2s, whereas the regulatory phosphorylation appears to be a unique property of JAK2. Finally, the co-crystal structure of TYK2 JH2 complexed with a small molecule inhibitor demonstrates that JH2 is accessible to ATP-competitive compounds, which offers novel approaches for targeting cytokine signaling as well as potential therapeutic applications.

Tricyclic covalent inhibitors selectively target Jak3 through an active site thiol

The action of Janus kinases (JAKs) is required for multiple cytokine signaling pathways, and as such, JAK inhibitors hold promise for treatment of autoimmune disorders, including rheumatoid arthritis, inflammatory bowel disease, and psoriasis. However, due to high similarity in the active sites of the four members (Jak1, Jak2, Jak3, and Tyk2), developing selective inhibitors within this family is challenging. We have designed and characterized substituted, tricyclic Jak3 inhibitors that selectively avoid inhibition of the other JAKs. This is accomplished through a covalent interaction between an inhibitor containing a terminal electrophile and an active site cysteine (Cys-909). We found that these ATP competitive compounds are irreversible inhibitors of Jak3 enzyme activity in vitro. They possess high selectivity against other kinases and can potently (IC50 < 100 nm) inhibit Jak3 activity in cell-based assays. These results suggest irreversible inhibitors of this class may be useful selective agents, both as tools to probe Jak3 biology and potentially as therapies for autoimmune diseases.

Important scaffold function of the Janus kinase 2 uncovered by a novel mouse model harboring a Jak2 activation-loop mutation

Janus kinases (Jak) play essential roles in cytokine and growth factor signaling. Conventional gene targeting of Jak2, creating a null allele, leads to a block in definitive erythropoiesis as a result of failing signal transduction at the homomeric erythropoietin receptor (EpoR) and at the heteromeric interferon γ receptor (IFNGR). To investigate the in vivo relevance of the activation loop of Jak2, a Jak2-YY1007/1008FF knockin mutation was introduced into the germline of mice. The phenotype of the Jak2(FF/FF) mouse line reveals that tyrosine residues 1007/1008 are absolutely essential for kinase function and signal transduction at the homomeric EpoR. Detailed studies using the Jak2 activation loop mutant uncover an essential scaffolding function of Jak2 within the IFNGR receptor complex and reveal that Jak1 can mediate a semi-redundant function for IFNGR signal transduction. These studies are highly important for the molecular understanding of cytokine and growth factor signaling and provide new insights for future strategies in the design of pharmacological blockers of Jak2.

Pharmacological activation of AMPK suppresses inflammatory response evoked by IL-6 signalling in mouse liver and in human hepatocytes

Interleukin-6 (IL-6) induces inflammatory signalling in liver, leading to impaired insulin action in hepatocytes. In this study, we demonstrate that pharmacological activation of AMP-activated protein kinase (AMPK) represses IL-6-stimulated expression of proinflammatory markers serum amyloid A (Saa) as well as suppressor of cytokine signalling 3 (Socs3) in mouse liver. Further studies using the human hepatocellular carcinoma cell line HepG2 suggest that AMPK inhibits IL-6 signalling by repressing IL-6-stimulated phosphorylation of several downstream components of the pathway such as Janus kinase 1 (JAK1), SH2-domain containing protein tyrosine phosphatase 2 (SHP2) and signal transducer and activator of transcription 3 (STAT3). In summary, inhibition of IL-6 signalling cascade in liver by the metabolic master switch of the body, AMPK, supports the role of this kinase as a crucial point of convergence of metabolic and inflammatory pathways in hepatocytes.

Enabling structure-based drug design of Tyk2 through co-crystallization with a stabilizing aminoindazole inhibitor

Background

Structure-based drug design (SBDD) can accelerate inhibitor lead design and optimization, and efficient methods including protein purification, characterization, crystallization, and high-resolution diffraction are all needed for rapid, iterative structure determination. Janus kinases are important targets that are amenable to structure-based drug design. Here we present the first mouse Tyk2 crystal structures, which are complexed to 3-aminoindazole compounds.

Results

A comprehensive construct design effort included N- and C-terminal variations, kinase-inactive mutations, and multiple species orthologs. High-throughput cloning and expression methods were coupled with an abbreviated purification protocol to optimize protein solubility and stability. In total, 50 Tyk2 constructs were generated. Many displayed poor expression, inadequate solubility, or incomplete affinity tag processing. One kinase-inactive murine Tyk2 construct, complexed with an ATP-competitive 3-aminoindazole inhibitor, provided crystals that diffracted to 2.5–2.6 Å resolution. This structure revealed initial “hot-spot” regions for SBDD, and provided a robust platform for ligand soaking experiments. Compared to previously reported human Tyk2 inhibitor crystal structures (Chrencik et al. (2010) J Mol Biol 400:413), our structures revealed a key difference in the glycine-rich loop conformation that is induced by the inhibitor. Ligand binding also conferred resistance to proteolytic degradation by thermolysin. As crystals could not be obtained with the unliganded enzyme, this enhanced stability is likely important for successful crystallization and inhibitor soaking methods.

Conclusions

Practical criteria for construct performance and prioritization, the optimization of purification protocols to enhance protein yields and stability, and use of high-throughput construct exploration enable structure determination methods early in the drug discovery process. Additionally, specific ligands stabilize Tyk2 protein and may thereby enable crystallization.

Depletion of cellular glutathione modulates LIF-induced JAK1-STAT3 signaling in cardiac myocytes

Previously we reported that the sesquiterpene lactone parthenolide induces oxidative stress in cardiac myocytes, which blocks Janus kinase (JAK) activation by the interleukin 6 (IL-6)-type cytokines. One implication suggested by this finding is that IL-6 signaling is dependent upon cellular anti-oxidant defenses or redox status. Therefore, the present study was undertaken to directly test the hypothesis that JAK1 signaling by the IL-6-type cytokines in cardiac myocytes is impaired by glutathione (GSH) depletion, since this tripeptide is one of the major anti-oxidant molecules and redox-buffers in cells. Cardiac myocytes were pretreated for 6h with l-buthionine-sulfoximine (BSO) to inhibit GSH synthesis. After 24h, cells were dosed with the IL-6-like cytokine, leukemia inhibitory factor (LIF). BSO treatment decreased GSH levels and dose-dependently attenuated activation of JAK1, Signal Transducer and Activator of Transcription 3 (STAT3), and extracellular signal regulated kinases 1 and 2 (ERK1/2). Addition of glutathione monoethyl ester, which is cleaved intracellularly to GSH, prevented attenuation of LIF-induced JAK1 and STAT3 activation, as did the reductant N-acetyl-cysteine. Unexpectedly, LIF-induced STAT1 activation was unaffected by GSH depletion. Evidence was found that STAT3 is more resistant than STAT1 to intermolecular disulfide bond formation under oxidizing conditions and more likely to retain the monomeric form, suggesting that conformational differences explain the differential effect of GSH depletion on STAT1 and STAT3. Overall, our findings indicate that activation of both JAK1 and STAT3 is redox-sensitive and the character of IL-6 type cytokine signaling in cardiac myocytes is sensitive to changes in the cellular redox status. In cardiac myocytes, activation of STAT1 may be favored over STAT3 under oxidizing conditions due to GSH depletion and/or augmented reactive oxygen species production, such as in ischemia-reperfusion and heart failure.

Regulation of inflammation by interleukin-4: a review of "alternatives"

Studies of IL-4 have revealed a wealth of information on the diverse roles of this cytokine in homeostatic regulation and disease pathogenesis. Recent data suggest that instead of simple linear regulatory pathways, IL-4 drives regulation that is full of alternatives. In addition to the well-known dichotomous regulation of Th cell differentiation by IL-4, this cytokine is engaged in several other alternative pathways. Its own production involves alternative mRNA splicing, yielding at least two functional isoforms: full-length IL-4, encoded by the IL-4 gene exons 1-4, and IL-4δ2, encoded by exons 1, 3, and 4. The functional effects of these two isoforms are in some ways similar but in other ways quite distinct. When binding to the surface of target cells, IL-4 may differentially engage two different types of receptors. By acting on macrophages, a cell type critically involved in inflammation, IL-4 induces the so-called alternative macrophage activation. In this review, recent advances in understanding these three IL-4-related branch points--alternative splicing of IL-4, differential receptor engagement by IL-4, and differential regulation of macrophage activation by IL-4--are summarized in light of their contributions to inflammation.

ROCK and JAK1 signaling cooperate to control actomyosin contractility in tumor cells and stroma

Proinflammatory cytokines are frequently observed in the tumor microenvironment, and chronic inflammation is involved in cancer initiation and progression. We show that cytokine signaling through the receptor subunit GP130-IL6ST and the kinase JAK1 generates actomyosin contractility through Rho-kinase dependent signaling. This pathway generates contractile force in stromal fibroblasts to remodel the extracellular matrix to create tracks for collective migration of squamous carcinoma cells and provides the high levels of actomyosin contractility required for migration of individual melanoma cells in the rounded, "amoeboid" mode. Thus, cytokine signaling can generate actomyosin contractility in both stroma and tumor cells. Strikingly, actomyosin contractility itself positively modulates activity of the transcription factor STAT3 downstream of JAK1, demonstrating positive feedback within the signaling network.

Jak1 has a dominant role over Jak3 in signal transduction through γc-containing cytokine receptors

Genetic deficiency of Jak3 leads to abrogation of signal transduction through the common gamma chain (γc) and thus to immunodeficiency suggesting that specific inhibition of Jak3 kinase may result in immunosuppression. Jak1 cooperates with Jak3 in signaling through γc-containing receptors. Unexpectedly, a Jak3-selective inhibitor was less efficient in abolishing STAT5 phosphorylation than pan-Jak inhibitors. We therefore explored the roles of Jak1 and Jak3 kinase functionality in signaling using a reconstituted system. The presence of kinase-inactive Jak1 but not kinase-inactive Jak3 resulted in complete abolishment of STAT5 phosphorylation. Specific inhibition of the "analog-sensitive" mutant AS-Jak1 but not AS-Jak3 by the ATP-competitive analog 1NM-PP1 abrogated IL-2 signaling, corroborating the data with the selective Jak3 inhibitor. Jak1 thus plays a dominant role over Jak3 and these data challenge the notion that selective ATP-competitive Jak3 kinase inhibitors will be effective.

Jak/STAT pathways in cytokine signaling and myeloproliferative disorders: approaches for targeted therapies

Hematopoiesis is the cumulative result of intricately regulated signaling pathways that are mediated by cytokines and their receptors. Studies conducted over the past 10 to 15 years have revealed that hematopoietic cytokine receptor signaling is largely mediated by a family of tyrosine kinases termed Janus kinases (JAKs) and their downstream transcription factors, termed STATs (signal transducers and activators of transcription). Aberrations in these pathways, such as those caused by the recently identified JAK2(V617F) mutation and translocations of the JAK2 gene, are underlying causes of leukemias and other myeloproliferative disorders. This review discusses the role of JAK/STAT signaling in normal hematopoiesis as well as genetic abnormalities associated with myeloproliferative and myelodisplastic syndromes. This review also summarizes the status of several small molecule JAK2 inhibitors that are currently at various stages of clinical development. Several of these compounds appear to improve the quality of life of patients with myeloproliferative disorders by palliation of disease-related symptoms. However, to date, these agents do not seem to significantly affect bone marrow fibrosis, alter marrow histopathology, reverse cytopenias, reduce red cell transfusion requirements, or significantly reduce allele burden. These results suggest the possibility that additional mutational events might be associated with the development of these neoplasms, and indicate the need for combination therapies as the nature and significance of these additional molecular events is better understood.

Expression, purification, and characterization of TYK-2 kinase domain, a member of the Janus kinase family

The Janus kinase family consists of four members: JAK-1, -2, -3 and TYK-2. While JAK-2 and JAK-3 have been well characterized biochemically, there is little data on TYK-2. Recent work suggests that TYK-2 may play a critical role in the development of a number of inflammatory processes. We have carried out a series of biochemical studies to better understand TYK-2 enzymology and its inhibition profile, in particular how the TYK-2 phosphorylated forms differ from each other and from the other JAK family members. We have expressed and purified milligram quantities of the TYK-2 kinase domain (KD) to high purity and developed a method to separate the non-, mono- (pY(1054)) and di-phosphorylated forms of the enzyme. Kinetic studies (k(cat(app))/K(m(app))) indicated that phosphorylation of the TYK-2-KD (pY(1054)) increased the catalytic efficiency 4.4-fold compared to its non-phosphorylated form, while further phosphorylation to generate the di-phosphorylated enzyme imparted no further increase in activity. These results are in contrast to those obtained with the JAK-2-KD and JAK-3-KD, where little or no increase in activity occurred upon mono-phosphorylation, while di-phosphorylation resulted in a 5.1-fold increase in activity for the JAK-2-KD. Moreover, ATP-competitive inhibitors demonstrated 10-30-fold shifts in potency (K(i(app))) as a result of the TYK-2-KD phosphorylation state, while the shifts for JAK-3-KD were only 2-3-fold and showed little or no change for JAK-2-KD. Thus, the phosphorlyation state imparted differential effects on both activity and inhibition within the JAK family of kinases.

The Interleukin (IL)‐2 Family Cytokines: Survival and Proliferation Signaling Pathways in T Lymphocytes

Lymphocyte populations in the immune system are maintained by a well-organized balance between cellular proliferation, cellular survival and programmed cell death (apoptosis). One of the primary functions of many cytokines is to coordinate these processes. In particular, the interleukin (IL)-2 family of cytokines, which consists of six cytokines (IL-2, IL-4, IL-7, IL-9, IL-15 and IL-21) that all share a common receptor subunit (gammac), plays a major role in promoting and maintaining T lymphocyte populations. The details of the molecular signaling pathways mediated by these cytokines have not been fully elucidated. However, the three major pathways clearly involved include the JAK/STAT, MAPK and phosphatidylinositol 3-kinase (P13K) pathways. The details of these pathways as they apply to the IL-2 family of cytokines is discussed, with a focus on their roles in proliferation and survival signaling.

Combinatorial Signal Transduction Responses Mediated by Interleukin-2 and -4 Receptors in a Helper TH2 Cell Line

The cytokines interleukin (IL)-2 and IL-4 are important regulators of the adaptive immune response, due in part to their effects on clonal expansion and differentiation of T cells. When IL-2 and IL-4 are administered together, both antagonistic and synergistic effects have been reported, but little is known in general concerning the mechanisms underlying such combinatorial effects. We found evidence for both effects in the proliferation responses of the IL-2 and IL-4 responsive T cell line, HT-2; IL-4 delays the onset of cell growth yet ultimately allows a higher cell density to be achieved in static culture. At the level of signal transduction pathways, we found that IL-4 partially inhibits IL-2 receptor-mediated pathways (PI3K/Akt, Ras/Erk, and STAT5a/b) and does not prolong their transient kinetics. This mode of antagonism, but not the effects on cell proliferation, is overcome at higher concentrations of IL-2 that are sufficient to saturate the signaling responses. By comparison, IL-4-stimulated activation of STAT6 is unaffected by IL-2 and shows sustained kinetics, and we speculate that this or another IL-4 receptor-specific pathway is responsible for the effects of IL-4 on IL-2-stimulated proliferation. A possibly related observation is that IL-4 induces a dramatic cell adhesion phenotype.

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.

Historical developments in the research of interferon receptors

Interferons (IFNs) were discovered 50 years ago independently by Isaacs and Lindemann and by Nagata and Kojima. When it was later realized that IFNs are active at very low concentrations, research began to determine how their powerful effects were generated from such a small initial signal. It has since been established that interferons, as well as all other cytokines, employ cell surface receptors to translate their presence in the serum to a potent cellular response to a viral infection. These receptor complexes are composed of multiple distinct glycosylated transmembrane polypeptides, a number of protein tyrosine kinases, and interact transiently with a large variety of other proteins including transcription factors, phosphatases, signaling repressors, and adaptor proteins coupling the receptor to alternative signaling pathways. Three major receptor complexes exist that are exclusive to each of three major classes of interferon. Even though the effects of each major class of interferon vary physiologically, each receptor complex interacts with its ligand in similar ways and activates similar signaling cascades. In this mini-review, we take a historical perspective at the major events in the characterization of interferon receptors, discussing interesting results that still need to be explained.

The regulation of Foxp3 expression in regulatory CD4(+)CD25(+)T cells: multiple pathways on the road

Regulatory T cells (Treg cells) have been well documented to have a crucial physiological role in preventing the development of autoimmune diseases and keeping self-tolerance. Foxp3, a recently identified member of the forkhead transcription factors, serves as a master regulator for the development and function of CD4(+)CD25(+)Treg cells. Though it is well defined that Foxp3 expression is sufficient to program CD4(+)CD25(+)Treg cell development, the physiological factors initiating intracellular Foxp3 expression remain poorly understood so far. In the present manuscript, we try to summarize the recent advances regarding the regulatory roles of T-cell receptor (TCR), co-stimulatory molecules, interleukin-2 (IL-2), transforming growth factor-beta (TGF-beta) and beyond pathways on Foxp3 expression.

Cytokine receptor signalling and aging

With ageing the immune system is deregulated and this leads to the development of immunosenescence mainly affecting the adaptive immune response. There is much knowledge accumulated concerning various receptor functions and signalling with ageing such as TCR, FcRs, TLRs. Cytokines are playing a major role in haematopoietic cell functions and in the harmonious and integrated coordination of the innate and adaptive immune response. There exists a large amount of data on cytokine production changes with ageing, as IL-2 production is decreasing, while IL-6 production is increasing. In contrast, there is only scarce knowledge concerning the cytokine receptors and their signalling in ageing. However, there is some evidence that the signalling of IL-2 receptors is altered in T cells and macrophages, mainly in relation to the JAK/STAT pathway. We present here evidence that the IL-6 induced signalling is also altered in T cells with ageing. An alteration in the JAKs and STATs activations in T cells and macrophages was demonstrated. The exact cause of these altered activations is not known and future studies are needed to elucidate them. In this review we summarise our present knowledge on cytokine signalling with ageing, mainly focusing on IL-2 and IL-6 receptors signalling.

Janus kinase 3: a novel target for selective transplant immunosupression

Existing immunosuppressants inhibit lymphocyte activation and T cell cytokine signal transduction pathways, reducing the rate of acute rejection episodes to < 10%. However, the widespread tissue distribution of their molecular targets engenders pleiotropic toxicities. One strategy to address this problem seeks to identify compounds that selectively inhibit a target restricted in distribution to the lymphoid system. Janus kinase (Jak) 3 is such a molecule; it mediates signal transduction via the gamma common chain of lymphokine surface receptors. Disruption of this lymphoid-restricted enzyme would not be predicted to produce collateral damage in other organ systems. Development of selective Jak3 inhibitors has been difficult due to crossreactivity with its homologue, Jak2. In contrast to all other putative antagonists, which are discussed in detail herein, one Jak3 inhibitor, NC1153, shows at least 40-fold greater selective inhibition for Jak3 than for Jak2, is robustly synergistic with calcineurin antagonists, and, either alone or in combination with cyclosporin, produces no adverse effects in rodents preconditioned to be at heightened risk for nephrotoxicity, bone marrow suppression, or altered lipid metabolism.

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.

Adenosine acts through A2 receptors to inhibit IL-2-induced tyrosine phosphorylation of STAT5 in T lymphocytes: role of cyclic adenosine 3',5'-monophosphate and phosphatases

Adenosine is a purine nucleoside with immunosuppressive activity that acts through cell surface receptors (A(1), A(2a), A(2b), A(3)) on responsive cells such as T lymphocytes. IL-2 is a major T cell growth and survival factor that is responsible for inducing Jak1, Jak3, and STAT5 phosphorylation, as well as causing STAT5 to translocate to the nucleus and bind regulatory elements in the genome. In this study, we show that adenosine suppressed IL-2-dependent proliferation of CTLL-2 T cells by inhibiting STAT5a/b tyrosine phosphorylation that is associated with IL-2R signaling without affecting IL-2-induced phosphorylation of Jak1 or Jak3. The inhibitory effect of adenosine on IL-2-induced STAT5a/b tyrosine phosphorylation was reversed by the protein tyrosine phosphatase inhibitors sodium orthovanadate and bpV(phen). Adenosine dramatically increased Src homology region 2 domain-containing phosphatase-2 (SHP-2) tyrosine phosphorylation and its association with STAT5 in IL-2-stimulated CTLL-2 T cells, implicating SHP-2 in adenosine-induced STAT5a/b dephosphorylation. The inhibitory effect of adenosine on IL-2-induced STAT5a/b tyrosine phosphorylation was reproduced by A(2) receptor agonists and was blocked by selective A(2a) and A(2b) receptor antagonists, indicating that adenosine was mediating its effect through A(2) receptors. Inhibition of STAT5a/b phosphorylation was reproduced with cell-permeable 8-bromo-cAMP or forskolin-induced activation of adenylyl cyclase, and blocked by the cAMP/protein kinase A inhibitor Rp-cAMP. Forskolin and 8-bromo-cAMP also induced SHP-2 tyrosine phosphorylation. Collectively, these findings suggest that adenosine acts through A(2) receptors and associated cAMP/protein kinase A-dependent signaling pathways to activate SHP-2 and cause STAT5 dephosphorylation that results in reduced IL-2R signaling in T cells.

Impaired TCR-mediated induction of Ki67 by naive CD4+ T cells is only occasionally corrected by exogenous IL-2 in HIV-1 infection

Perturbations in naive T cell homeostasis and function may play a major role in the immunodeficiency that accompanies HIV infection. By examining naive CD4(+) T cell function on a single cell basis, we provide evidence that these cells have significant qualitative defects in HIV disease. Ki67, a molecule expressed during cell cycle progression, is induced less efficiently among naive CD4(+) T cells from HIV-infected individuals following activation with anti-TCR Ab. The impairment in Ki67 expression is evident even when a separate function, CD62L down-modulation, is within normal ranges. Moreover, the defects in Ki67 induction are only sometimes corrected by the addition of rIL-2 to cell cultures. An initial assessment of IL-2 unresponsiveness in cells from selected HIV-infected individuals suggests that the defect is not a consequence of impaired IL-2R expression or IL-2R signaling capability. Qualitative defects in naive T cells that cannot be routinely corrected by IL-2 have significant implications for disease pathogenesis and for strategies using IL-2 as a vaccine adjuvant in HIV disease.

Expression of an active p110 catalytic subunit of phosphatidylinositol 3-kinase alters the proliferative capacity of interleukin-2 receptor signals

Activation of phosphatidylinositol 3-kinase (PI3K) is an early and essential step in interleukin-2 receptor (IL-2R) signalling, and plays an important role in regulating both cell survival and cellular proliferation. In the present study, we utilized Baf-B03 cells expressing mutated IL-2R to examine the contribution of PI3K to proliferative capacity. In this model IL-2-mediated induction of the downstream PI3K-dependent signalling molecule p70 S6 kinase was detected, but there was no proliferative response. Increasing the level of PI3K activity by transfection of an active form of the catalytic subunit, p110*, enabled the proliferative capacity of the mutated receptor. Whereas, in cells without p110*, IL-2 lacked the capacity to induce c-myc and to overcome an S-phase checkpoint, S-phase transition was restored by transfection of p110*, and this was accompanied by an increase in the c-myc response. Despite the presence of p110*, activity cells still required IL-2R-derived signals for proliferation, and IL-2Rbeta truncated at amino acid 350 were sufficient to provide this signalling activity. The data support a model in which the level of available PI3K can determine the cellular response to IL-2.

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.

Anti-apoptotic signaling by the interleukin-2 receptor reveals a function for cytoplasmic tyrosine residues within the common gamma (gamma c) receptor subunit

The interleukin-2 receptor (IL-2R) is composed of one affinity-modulating subunit (IL-2Ralpha) and two essential signaling subunits (IL-2Rbeta and gammac). Although most known signaling events are mediated through tyrosine residues located within IL-2Rbeta, no functions have yet been ascribed to gammac tyrosine residues. In this study, we describe a role for gammac tyrosines in anti-apoptotic signal transduction. We have shown previously that a tyrosine-deficient IL-2Rbeta chain paired with wild type gammac stimulated enhancement of bcl-2 mRNA in IL-2-dependent T cells, but it was not determined which region of the IL-2R or which pathway was activated to direct this signaling response. Here we show that up-regulation of Bcl-2 by an IL-2R lacking IL-2Rbeta tyrosine residues leads to increased cell survival after cytokine deprivation; strikingly, this survival signal does not occur in the absence of gammac tyrosine residues. These gammac-dependent signals are revealed only in the absence of IL-2Rbeta tyrosines, indicating that the IL-2R engages at least two distinct signaling pathways to regulate apoptosis and Bcl-2 expression. Mechanistically, the gammac-dependent signal requires activation of Janus kinases 1 and 3 and is sensitive to wortmannin, implicating phosphatidylinositol 3-kinase. Consistent with involvement of phosphatidylinositol 3-kinase, Akt can be activated via tyrosine residues on gammac. Thus, gammac mediates an anti-apoptotic signaling pathway through Akt which cooperates with signals from its partner chain, IL-2Rbeta.

Mechanism of Janus kinase 3-catalyzed phosphorylation of a Janus kinase 1 activation loop peptide

A high-affinity IL-2 receptor requires two Janus protein tyrosine kinases (JAKs) for IL-2 signal transduction: JAK1 and JAK3. Since transphosphorylation of the two kinases is presumed to occur after receptor engagement we examined the phosphorylation by recombinant JAK3 of a peptide substrate corresponding to the JAK1 activation loop (KAIETDKEYYTVKD), which has two adjacent tyrosines. Mass spectral analysis of the enzymatically phosphorylated peptide showed that the second tyrosine was phosphorylated at a 30-fold greater rate than the first tyrosine. Moreover, no doubly phosphorylated peptide was detected by this analysis. Kinetic analysis of the reactions of singly phosphorylated JAK1 activation loop peptides showed that phosphorylating the first or second tyrosine decreased the k(cat)/K(m) for the phosphorylation of the other 115- and 26-fold, respectively. Singly changing each side chain of the KEYYTV portion of the peptide to a methyl group (alanine) yielded substrates comparable to the wild-type sequences in all cases except that of the first or second tyrosine, which showed a 153- or 70-fold drop in k(cat)/K(m), respectively. Using libraries of immobilized peptides with all 20 naturally occurring amino acids substituted for Y9 or T11 showed that the JAK3 tolerated substitution at T11 but prefers large hydrophobic amino acids at Y9. These results show that JAK3 does not act processively on the JAK1 activation loop in vitro and illustrate the role of Y9 in the recognition of the preferred site of phosphorylation which is Y10.

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.

JAKs, STATs and Src kinases in hematopoiesis

Hematopoiesis is the cumulative result of intricately regulated signal transduction cascades that are mediated by cytokines and their cognate receptors. Proper culmination of these diverse signaling pathways forms the basis for an orderly generation of different cell types and aberrations in these pathways is an underlying cause for diseases such as leukemias and other myeloproliferative and lymphoproliferative disorders. Over the past decade, downstream signal transduction events initiated upon cytokine/growth factor stimulation have been a major focus of basic and applied biomedical research. As a result, several key concepts have emerged allowing a better understanding of the complex signaling processes. A group of transcription factors, termed signal transducers and activators of transcription (STATs) appear to orchestrate the downstream events propagated by cytokine/growth factor interactions with their cognate receptors. Similarly, cytoplasmic Janus protein tyrosine kinases (JAKs) and Src family of kinases seem to play a critical role in diverse signal transduction pathways that govern cellular survival, proliferation, differentiation and apoptosis. Accumulating evidence suggests that STAT protein activation may be mediated by members of both JAK and Src family members following cytokine/growth factor stimulation. In addition, JAK kinases appear to be essential for the phosphorylation of the cytokine receptors which results in the creation of docking sites on the receptors for binding of SH2-containing proteins such as STATs, Src-kinases and other signaling intermediates. Cell and tissue-specificity of cytokine action appears to be determined by the nature of signal transduction pathways activated by cytokine/receptor interactions. The integration of these diverse signaling cues from active JAK kinases, members of the Src-family kinases and STAT proteins, leads to cell proliferation, cell survival and differentiation, the end-point of the cytokine/growth factor stimulus.

Alternate signalling pathways from the interleukin-2 receptor

Interleukin-2 (IL-2) plays a major role in the proliferation of cell populations during an immune reaction. The beta(c) and gamma(c) subunits of the IL-2 receptor (IL-2R) are sufficient and necessary for signal transduction. Despite lacking known catalytic domains, receptor engagement leads to the activation of a diverse array protein tyrosine kinases (PTKs). In resting or anergised T cells, Jak3 is not activated. Signals arising from the PROX domain of the gamma(c) subunit activate p56(lck) (lck) leading to the induction of anti-apoptotic mechanisms. When Jak3 is activated, in primed T cells, other PTKs predominantly mediate the induction of anti-apoptotic mechanisms and drive cellular proliferation. This review intends to suggest a role for these differences within the context of the immune system.

Mutations in severe combined immune deficiency (SCID) due to JAK3 deficiency

During the last 10 years, an increasing number of genes have been identified whose abnormalities account for primary immunodeficiencies, with defects in development and/or function of the immune system. Among them is the JAK3-gene, encoding for a tyrosine kinase that is functionally coupled to cytokine receptors which share the common gamma chain. Defects of this gene cause an autosomal recessive form of severe combined immunodeficiency with almost absent T-cells and functionally defective B-cells (T(-)B(+) SCID). Herewith, we present molecular information on the first 27 unique mutations identified in the JAK3 gene, including clinical data on all of the 23 affected patients reported so far. A variety of mutations scattered throughout all seven functional domains of the protein, and with different functional effects, have been identified. Availability of a molecular screening test, based on amplification of genomic DNA, facilitates the diagnostic approach, and has permitted recognition that JAK3 deficiency may also be associated with atypical clinical and immunological features. Development of a structural model of the JAK3 kinase domain has allowed characterization of the functional effects of the various mutations. Most importantly, molecular analysis at the JAK3 locus results in improved genetic counseling, allows early prenatal diagnosis, and prompts appropriate treatment (currently based on hematopoietic stem cell transplantation) in affected families.

Signaling domains of the interleukin 2 receptor

Interleukin (IL-)2 and its receptor (IL-2R) constitute one of the most extensively studied cytokine receptor systems. IL-2 is produced primarily by activated T cells and is involved in early T cell activation as well as in maintaining homeostatic immune responses that prevent autoimmunity. This review focuses on molecular signaling pathways triggered by the IL-2/IL-2R complex, with an emphasis on how the IL-2R physically translates its interaction with IL-2 into a coherent biological outcome. The IL-2R is composed of three subunits, IL-2Ralpha, IL-2Rbeta and gammac. Although IL-2Ralpha is an important affinity modulator that is essential for proper responses in vivo, it does not contribute to signaling due a short cytoplasmic tail. In contrast, IL-2Rbeta and gammac together are necessary and sufficient for effective signal transduction, and they serve physically to connect the receptor complex to cytoplasmic signaling intermediates. Despite an absolute requirement for gammac in signaling, the majority of known pathways physically link to the receptor via IL-2Rbeta, generally through phosphorylated cytoplasmic tyrosine residues. This review highlights work performed both in cultured cells and in vivo that defines the functional contributions of specific receptor subdomains-and, by inference, the specific signaling pathways that they activate-to IL-2-dependent biological activities.