Janus kinases in cytokine signalling

Enzymatic Characterization of Wild-Type and Mutant Janus Kinase 1

Janus kinases (JAKs) are found constitutively associated with cytokine receptors and are present in an inactive state prior to cytokine exposure. Activating mutations of JAKs are causative for a number of leukemias, lymphomas, and myeloproliferative diseases. In particular, the JAK2V617F mutant is found in most human cases of polycythemia vera, a disease characterized by over-production of erythrocytes. The V617F mutation is found in the pseudokinase domain of JAK2 and it leads to cytokine-independent activation of the kinase, as does the orthologous mutation in other JAK-family members. The mechanism whereby this mutation hyperactivates these kinases is not well understood, primarily due to the fact that the full-length JAK proteins are difficult to produce for structural and kinetic studies. Here we have overcome this limitation to perform a series of enzymatic analyses on full-length JAK1 and its constitutively active mutant form (JAK1V658F). Consistent with previous studies, we show that the presence of the pseudokinase domain leads to a dramatic decrease in enzymatic activity with no further decrease from the presence of the FERM or SH2 domains. However, we find that the mutant kinase, in vitro, is indistinguishable from the wild-type enzyme in every measurable parameter tested: KM (ATP), KM (substrate), kcat, receptor binding, thermal stability, activation rate, dephosphorylation rate, and inhibitor affinity. These results show that the V658F mutation does not enhance the intrinsic enzymatic activity of JAK. Rather this data is more consistent with a model in which there are cellular processes and interactions that prevent JAK from being activated in the absence of cytokine and it is these constraints that are affected by disease-causing mutations.

miR-4324-RACGAP1-STAT3-ESR1 feedback loop inhibits proliferation and metastasis of bladder cancer

Considering the importance of microRNAs (miRNAs) in regulating cellular processes, we performed microarray analysis and revealed miR-4324 as one of the most differentially expressed miRNAs in bladder cancer (BCa). Then, we discovered that miR-4324 was a negative regulator of Rac GTPase activating protein 1 (RACGAP1) and that RACGAP1 functioned as an oncogenic protein in BCa. Our studies indicated that ectopic overexpression of miR-4324 in BCa cells significantly suppressed cell proliferation and metastasis and enhanced chemotherapy sensitivity to doxorubicin by repressing RACGAP1 expression. Further studies showed that estrogen receptor 1 (ESR1) increased the expression of miR-4324 by binding to its promoter, while the downregulation of ESR1 in BCa was caused by hypermethylation of its promoter. p-STAT3 induced the enrichment of DNMT3B by binding to the ESR1 promoter and then induced methylation of the ESR1 promoter. In turn, RACGAP1 induced STAT3 phosphorylation, increasing p-STAT3 expression and promoting its translocation to the nucleus. Therefore, the miR-4324-RACGAP1-STAT3-ESR1 feedback loop could be a critical regulator of BCa progression.

Food for thought: Leptin regulation of hippocampal function and its role in Alzheimer's disease

Accumulating evidence indicates that diet and body weight are important factors associated with Alzheimer's disease (AD), with a significant increase in AD risk linked to mid-life obesity, and weight loss frequently occurring in the early stages of AD. This has fuelled interest in the hormone leptin, as it is an important hypothalamic regulator of food intake and body weight, but leptin also markedly influences the functioning of the hippocampus; a key brain region that degenerates in AD. Increasing evidence indicates that leptin has cognitive enhancing properties as it facilitates the cellular events that underlie hippocampal-dependent learning and memory. However, significant reductions in leptin's capacity to regulate hippocampal synaptic function occurs with age and dysfunctions in the leptin system are associated with an increased risk of AD. Moreover, leptin is a potential novel target in AD as leptin treatment has beneficial effects in various models of AD. Here we summarise recent advances in leptin neurobiology with particular focus on regulation of hippocampal synaptic function by leptin and the implications of this for neurodegenerative disorders like AD. This article is part of the Special Issue entitled 'Metabolic Impairment as Risk Factors for Neurodegenerative Disorders.'

Layered double hydroxide nanoparticles promote self-renewal of mouse embryonic stem cells through the PI3K signaling pathway

Embryonic stem cells (ESCs) hold great potential for regenerative medicine due to their two unique characteristics: self-renewal and pluripotency. Several groups of nanoparticles have shown promising applications in directing the stem cell fate. Herein, we investigated the cellular effects of layered double hydroxide nanoparticles (LDH NPs) on mouse ESCs (mESCs) and the associated molecular mechanisms. Mg-Al-LDH NPs with an average diameter of ∼100 nm were prepared by hydrothermal methods. To determine the influences of LDH NPs on mESCs, cellular cytotoxicity, self-renewal, differentiation potential, and the possible signaling pathways were explored. Evaluation of cell viability, lactate dehydrogenase release, ROS generation and apoptosis demonstrated the low cytotoxicity of LDH NPs. The alkaline phosphatase activity and the expression of pluripotency genes in mESCs were examined, which indicated that exposure to LDH NPs could support self-renewal and inhibit spontaneous differentiation of mESCs under feeder-free culture conditions. The self-renewal promotion was further proved to be independent of the leukemia inhibitory factor (LIF). Furthermore, cells treated with LDH NPs maintained the potential to differentiate into all three germ layers both in vitro and in vivo through formation of embryoid bodies and teratomas. In addition, we observed that LDH NPs initiated the activation of the PI3K/Akt pathway, while treatment with the PI3K inhibitor LY294002 could block the effects of LDH NPs on mESCs. The results confirmed that the promotion of self-renewal by LDH NPs was associated with activation of the PI3K/Akt signaling pathway. Altogether, our studies identified a new role of LDH NPs in maintaining self-renewal of mouse ES cells which could potentially be applied in stem cell research.

Characterization and regulation of mechanical loading-induced compensatory muscle hypertrophy

In mammalian systems, skeletal muscle exists in a dynamic state that monitors and regulates the physiological investment in muscle size to meet the current level of functional demand. This review attempts to consolidate current knowledge concerning development of the compensatory hypertrophy that occurs in response to a sustained increase in the mechanical loading of skeletal muscle. Topics covered include: defining and measuring compensatory hypertrophy, experimental models, loading stimulus parameters, acute responses to increased loading, hyperplasia, myofiber-type adaptations, the involvement of satellite cells, mRNA translational control, mechanotransduction, and endocrinology. The authors conclude with their impressions of current knowledge gaps in the field that are ripe for future study.

Chicken interferons, their receptors and interferon-stimulated genes

The prevalence of pathogenic viruses is a serious issue as they pose a constant threat to both the poultry industry and to human health. To prevent these viral infections an understanding of the host-virus response is critical, especially for the development of novel therapeutics. One approach in the control of viral infections would be to boost the immune response through administration of cytokines, such as interferons. However, the innate immune response in chickens is poorly characterised, particularly concerning the interferon pathway. This review will provide an overview of our current understanding of the interferon system of chickens, including their cognate receptors and known interferon-stimulated gene products.

Role of the IL-6-JAK1-STAT3-Oct-4 pathway in the conversion of non-stem cancer cells into cancer stem-like cells

Previous studies have demonstrated that a small subset of cancer cells is capable of tumor initiation. The existence of tumor initiating cancer stem cells (CSCs) has several implications in terms of future cancer treatment and therapies. However, recently, several researchers proposed that differentiated cancer cells (non-CSCs) can convert to stem-like cells to maintain equilibrium. These results imply that removing CSCs may prompt non-CSCs in the tumor to convert into stem cells to maintain the equilibrium. Interleukin-6 (IL-6) has been found to play an important role in the inducible formation of CSCs and their dynamic equilibrium with non-stem cells. In this study, we used CSC-like human breast cancer cells and their alternate subset non-CSCs to investigate how IL-6 regulates the conversion of non-CSCs to CSCs. MDA-MB-231 and MDA-MB-453 CSC-like cells formed mammospheres well, whereas most of non-stem cells died by anoikis and only part of the remaining non-stem cells produced viable mammospheres. Similar results were observed in xenograft tumor formation. Data from cytokine array assay show that IL-6 was secreted from non-CSCs when cells were cultured in ultra-low attachment plates. IL-6 regulates CSC-associated OCT-4 gene expression through the IL-6-JAK1-STAT3 signal transduction pathway in non-CSCs. Inhibiting this pathway by treatment with anti-IL-6 antibody (1 μg/ml) or niclosamide (0.5-2 μM)/LLL12 (5-10 μM) effectively prevented OCT-4 gene expression. These results suggest that the IL-6-JAK1-STAT3 signal transduction pathway plays an important role in the conversion of non-CSCs into CSCs through regulation of OCT-4 gene expression.

Influence of Tyrphostin AG490 on the expression of diabetes-associated markers in human adipocytes

Tyrosine kinase inhibitors (TKi) hold promise as a treatment for a variety of disorders ranging from those in oncology to diseases thought as immune mediated. Tyrphostin AG490 is a potent Jak-Stat TKi shown effective in the prevention of allograft transplant rejection, experimental autoimmune disease, as well as the treatment of cancer. However, given its ability to modulate this important but pleiotropic intracellular pathway, we thought that it is important to examine its effects on glucose metabolism and expression of major transcription factors and adipokines associated with insulin insensitivity and diabetes. We investigated the metabolic effects of AG490 on glucose levels in vivo using an animal model of diabetes, nonobese diabetic (NOD) mice, and transcription factor expression through assessment of human adipocytes. AG490 treatment of young nondiabetic NOD mice significantly reduced blood glucose levels (p = 0.002). In vitro, treatment of adipocytes with rosiglitazone, an insulin sensitizer that binds to peroxisome proliferator-activated receptor (PPAR) receptors and increases the adipocyte response to insulin, significantly increased the expression of the antidiabetic adipokine adiponectin. Importantly, the combination of rosiglitazone plus Tyrphostin AG490 further increased this effect and was specifically associated with significant upregulation of C-enhanced binding protein (C/EBP) (p < 0.0001). In terms of the mechanism underlying this action, regulatory regions of the PPARγ, ADIPOQ, and C/EBP contain the Stat5 DNA-binding sequences and were demonstrated, by gel shift experiments in vitro. These data suggest that blocking Jak-Stat signaling with AG490 reduces blood glucose levels and modulates the expression of transcription factors previously associated with diabetes, thereby supporting its potential as a therapy for this disease.

The tyrphostin agent AG490 prevents and reverses type 1 diabetes in NOD mice

BACKGROUND

Recent studies in the NOD (non-obese diabetic) mouse model of type 1 diabetes (T1D) support the notion that tyrosine kinase inhibitors have the potential for modulating disease development. However, the therapeutic effects of AG490 on the development of T1D are unknown.

MATERIALS AND METHODS

Female NOD mice were treated with AG490 (i.p, 1 mg/mouse) or DMSO starting at either 4 or 8 week of age, for five consecutive week, then once per week for 5 additional week. Analyses for the development and/or reversal of diabetes, insulitis, adoptive transfer, and other mechanistic studies were performed.

RESULTS

AG490 significantly inhibited the development of T1D (p = 0.02, p = 0.005; at two different time points). Monotherapy of newly diagnosed diabetic NOD mice with AG490 markedly resulted in disease remission in treated animals (n = 23) in comparision to the absolute inability (0%; 0/10, p = 0.003, Log-rank test) of DMSO and sustained eugluycemia was maintained for several months following drug withdrawal. Interestingly, adoptive transfer of splenocytes from AG490 treated NOD mice failed to transfer diabetes to recipient NOD.Scid mice. CD4 T-cells as well as bone marrow derived dendritic cells (BMDCs) from AG490 treated mice, showed higher expression of Foxp3 (p<0.004) and lower expression of co-stimulatory molecules, respectively. Screening of the mouse immune response gene arrary indicates that expression of costimulaotry molecule Ctla4 was upregulated in CD4+ T-cell in NOD mice treated with AG490, suggesting that AG490 is not a negative regulator of the immune system.

CONCLUSION

The use of such agents, given their extensive safety profiles, provides a strong foundation for their translation to humans with or at increased risk for the disease.

Promotion of self-renewal of embryonic stem cells by midkine

AIM

To characterize the expression and function of midkine (MK) in an in vitro embryonic stem cell (ESC) culture system.

METHODS

To investigate the potential roles of MK, the expression of MK in ESCs was evaluated by RT-PCR and immunocytochemistry. The effects of MK on the self-renewal of ESCs were measured using alkaline phosphatase assays, immunocytochemistry, RT-PCR and colony-forming assays. The mechanism of the growth-promoting effect of MK in mESCs was assessed by cell cycle analysis and Western blot analysis.

RESULTS

MK is expressed in mouse embryonic stem cells (mESCs), human embryonic stem cells (hESCs) and mouse embryonic fibroblasts (MEFs). MK promotes proliferation and self-renewal of mESCs both in feeder and feeder free culture systems. It also promotes self-renewal and proliferation of hESCs. Further study showed that MK promotes the growth of mESCs by inhibiting apoptosis while accelerating the progression toward the S phase, and enhances mESC self-renewal through PI3K/Akt signaling pathway.

CONCLUSION

MK plays profound roles in ESCs. MK/PTPzeta signaling pathway is a novel pathway in the signal network maintaining pluripotency of ESCs. The results extend our knowledge on pluripotency control of ESCs and the relationship between ESCs and cancers.

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.

Localization of type I interferon receptor limits interferon-induced TLR3 in epithelial cells

Previous studies have shown that influenza infections increase Toll-like receptor 3 (TLR3) expression and that type I interferons (IFNs) may play a role in this response. This study aimed to expand on the role of type I IFNs in the influenza-induced upregulation of TLR3 and determine whether and how the localization of the IFN-alpha/beta receptor (IFNAR) in respiratory epithelial cells could modify IFN-induced responses. Using differentiated primary human airway epithelial cells this study demonstrates that soluble mediators secreted in response to influenza infection upregulate TLR3 expression in naive cells. This response was associated with an upregulation of type I IFNs and stimulation with type I, but not type II, IFNs enhanced TLR3 expression. Interestingly, although influenza infection results in IFN-beta release both toward the apical and basolateral sides of the epithelium, TLR3 expression is only enhanced in cells stimulated with IFN-beta from the basolateral side. Immunohistochemical analysis demonstrates that IFNAR expression is limited to the basolateral side of differentiated human airway epithelial cells. However, non- or poorly differentiated epithelial cells express IFNAR more toward the apical side. These data demonstrate that restricted expression of the IFNAR in the differentiated airway epithelium presents a potential mechanism of regulating type I IFN-induced TLR3 expression.

A Rac GTPase-activating protein, MgcRacGAP, is a nuclear localizing signal-containing nuclear chaperone in the activation of STAT transcription factors

In addition to their pleiotropic functions under physiological conditions, transcription factors STAT3 and STAT5 also have oncogenic activities, but how activated STATs are transported to the nucleus has not been fully understood. Here we show that an MgcRacGAP mutant lacking its nuclear localizing signal (NLS) blocks nuclear translocation of p-STATs both in vitro and in vivo. Unlike wild-type MgcRacGAP, this mutant did not promote complex formation of phosphorylated STATs (p-STATs) with importin alpha in the presence of GTP-bound Rac1, suggesting that MgcRacGAP functions as an NLS-containing nuclear chaperone. We also demonstrate that mutants of STATs lacking the MgcRacGAP binding site (the strand betab) are hardly tyrosine phosphorylated after cytokine stimulation. Intriguingly, mutants harboring small deletions in the C'-adjacent region (betab-betac loop region) of the strand betab became constitutively active with the enhanced binding to MgcRacGAP. The molecular basis of this phenomenon will be discussed, based on the computer-assisted tertiary structure models of STAT3. Thus, MgcRacGAP functions as both a critical mediator of STAT's tyrosine phosphorylation and an NLS-containing nuclear chaperone of p-STATs.

Negative regulation of Jak2 by its auto-phosphorylation at tyrosine 913 via the Epo signaling pathway

Janus kinase 2 (Jak2) has a pivotal role in erythropoietin (Epo) signaling pathway, including erythrocyte differentiation and Stat5 activation. In the course of screening for critical phosphorylation of tyrosine residues in Jak2, we identified tyrosine 913 (Y(913)) as a novel and functional phosphorylation site, which negatively regulates Jak2. Phosphorylation at Y(913) rapidly occurred and was sustained for at least 120 min after Epo stimulation, in contrast to the transient phosphorylation of Y(1007/1008) in the activation loop of Jak2. Interestingly, phosphorylation defective mutation of Y(913) (Y(913)F) results in a significant enhancement of Epo-induced Jak2 activation, whereas phosphorylation mimic mutation of Y(913) (Y(913)E) completely abrogated its activation. Furthermore, Jak2 deficient fetal liver cells expressing Y(913)F mutant generated many mature erythroid BFU-E and CFU-E colonies, while Y(913)E mutant failed to reconstitute Jak2 deficiency. We also demonstrate, in Jak1, phosphorylation of Y(939), a corresponding tyrosine residue with Y(913), negatively regulated Jak1 signaling pathway. Accordingly, our results suggest that this tyrosine phosphorylation in JH1 domain may be involved in common negative regulation mechanism for Jak family.

Key molecules involved in receptor-mediated lymphocyte activation

This unit, along with Unit 11.9B, provides a summary of our current knowledge about various signaling pathways critical to the function of immune cells. Here, our understanding of T cell receptor (TCR)- and B cell receptor (BCR)-mediated signaling is summarized. A schematic representation of immunologically relevant cytokine receptors and the Janus Family Kinases (JAKs) that is activated through these receptors is provided, along with details about molecules involved in interleukin 2 mediated signal transduction.

Leptin affects endocardial cushion formation by modulating EMT and migration via Akt signaling cascades

Blood circulation is dependent on heart valves to direct blood flow through the heart and great vessels. Valve development relies on epithelial to mesenchymal transition (EMT), a central feature of embryonic development and metastatic cancer. Abnormal EMT and remodeling contribute to the etiology of several congenital heart defects. Leptin and its receptor were detected in the mouse embryonic heart. Using an ex vivo model of cardiac EMT, the inhibition of leptin results in a signal transducer and activator of transcription 3 and Snail/vascular endothelial cadherin-independent decrease in EMT and migration. Our data suggest that an Akt signaling pathway underlies the observed phenotype. Furthermore, loss of leptin phenocopied the functional inhibition of alphavbeta3 integrin receptor and resulted in decreased alphavbeta3 integrin and matrix metalloprotease 2, suggesting that the leptin signaling pathway is involved in adhesion and migration processes. This study adds leptin to the repertoire of factors that mediate EMT and, for the first time, demonstrates a role for the interleukin 6 family in embryonic EMT.

Signaling molecules involved in IFN-gamma-inducible nitric oxide synthase expression in the mouse trophoblast

PROBLEM

We have previously shown that trophoblast can generate nitric oxide (NO) and express inducible isoform of nitric oxide synthase (iNOS). Moreover, this production was changed by the presence of interferon-gamma (IFN-gamma) establishing a relationship between trophoblast inductive response and this proinflammatory cytokine.

METHOD OF STUDY

As the intracellular signal transduction pathway used by IFN-gamma in target cells is the Janus kinase (JAK)-signal transducer and transcription activator (STAT), here we analyzed in the mouse trophoblast the effect of IFN-gamma and staurosporine on mRNA and protein expressions of IFN-gamma signaling molecules correlating them with iNOS expression.

RESULTS

Interferon-gamma induced iNOS expression and upregulated Jaks and Stat1, but not Stat2 transcriptions. The protein distribution matched the mRNA expression pattern. These effects were abrogated when IFN-gamma receptor was blocked by staurosporine.

CONCLUSION

Due to the biological effects of NO-iNOS generated on induction of apoptosis and inflammatory responses, interaction between iNOS expression and IFN-gamma-mediated signaling is very important for understanding the physiology of trophoblast at the maternal-fetal interface. Our data indicate IFN-gamma acts specifically on trophoblast, regulating the expression of signaling molecules and is fundamental for iNOS expression.

Bruton's tyrosine kinase prevents activation of the anti-apoptotic transcription factor STAT3 and promotes apoptosis in neoplastic B-cells and B-cell precursors exposed to oxidative stress

Bruton's tyrosine kinase (BTK) was previously demonstrated to be a mediator of oxidative stress-induced apoptosis in irradiated neoplastic B-cells and B-cell precursors. Defective BTK expression in leukaemic B-cell precursors from infants with t(4;11) acute lymphoblastic leukaemia has been associated with radiation resistance. The present study examined whether BTK mediates apoptosis during oxidative stress by interfering with the anti-apoptotic function of signal transducer and activator of transcription 3 (STAT3). BTK physically associated with and tyrosine phosphorylated STAT3; this association was promoted by pervanadate (PV)-induced oxidative stress. The BTK/STAT3 interaction appeared to prevent STAT3 response to oxidative stress, because PV-induced STAT3 activation was markedly enhanced in DT40 chicken lymphoma B-cells that were rendered BTK-deficient by targeted disruption of the btk gene as well as in BTK-deficient RAMOS-1 human lymphoma B-cells. These BTK-deficient cells were highly resistant to oxidative stress-induced apoptosis triggered by PV treatment. Reconstitution of BTK-deficient DT40 cells with wild-type human BTK gene eliminated the amplification of the STAT3 response and restored the PV-induced apoptotic signal. Similarly, while the BTK-positive NALM-6 human leukaemic B-cell precursor cell line showed no STAT3 activation after PV treatment and was exquisitely sensitive to PV-induced apoptosis, PV failed to induce apoptosis in BTK-deficient RAMOS-1 human lymphoma B-cells that showed a robust STAT3 response. These results provide unprecedented biochemical and genetic evidence for a unique mode of cross-talk that occurs between BTK and STAT3 pathways during oxidative stress, whereby BTK may trigger apoptosis via negative regulation of the anti-apoptotic STAT3 activity.

Suppression of ES cell differentiation by retinol (vitamin A) via the overexpression of Nanog

Embryonic stem cells (ESCs) derived from the inner cell mass of blastocysts maintain their pluripotency through a complex interplay of different signaling pathways and transcription factors including Leukemia Inhibitory Factor (LIF), homeo-domain protein Nanog and POU-domain-containing transcription factor Oct3/4. LIF can maintain the self-renewal of mouse ESCs by activating the Jak/Stat3 pathway; however, it is dispensable for human ESCs. Nanog, a homeo-domain transcription factor alone is sufficient for sustaining the self-renewal of ESCs. Overexpression of Nanog by heterologous promoters can maintain self-renewal of human and mouse ESCs in the absence of LIF/Stat3 pathway. The mechanisms that control the expression of Nanog, however, remain poorly understood. In this report we demonstrate that retinol, the alcohol form of Vitamin A, can suppress the differentiation of ESCs by up-regulating the expression of Nanog. Retinol is mainly associated with differentiation through its active metabolite retinoic acid during early development of the embryo. The activation of Nanog by retinol is not mediated via retinoic acid signaling and appears to be independent of previously described LIF/Stat3, bone morphogenic proteins, Wnt/beta-catenin, and Oct3/4-Sox2 pathways. These studies therefore, reveal a previously unknown function of retinol and offer a model system to define alternate regulatory pathways that control the self-renewal of ESCs as well as to identify upstream "master" regulatory factors that are responsible for maintaining the integrity of stem cells.

Rac1 and a GTPase-activating protein, MgcRacGAP, are required for nuclear translocation of STAT transcription factors

STAT transcription factors are tyrosine phosphorylated upon cytokine stimulation and enter the nucleus to activate target genes. We show that Rac1 and a GTPase-activating protein, MgcRacGAP, bind directly to p-STAT5A and are required to promote its nuclear translocation. Using permeabilized cells, we find that nuclear translocation of purified p-STAT5A is dependent on the addition of GTP-bound Rac1, MgcRacGAP, importin α, and importin β. p-STAT3 also enters the nucleus via this transport machinery, and mutant STATs lacking the MgcRacGAP binding site do not enter the nucleus even after phosphorylation. We conclude that GTP-bound Rac1 and MgcRacGAP function as a nuclear transport chaperone for activated STATs.

Receptor specific downregulation of cytokine signaling by autophosphorylation in the FERM domain of Jak2

The tyrosine kinase, Janus kinase-2 (Jak2), plays a pivotal role in signal transduction through a variety of cytokine receptors, including the receptor for erythropoietin (Epo). Although the physiological relevance of Jak2 has been definitively established, less is known about its regulation. In studies assessing the roles of sites of tyrosine phosphorylation, we identified Y(119) in the FERM (band 4.1, Ezrin, radixin and moesin) domain as a phosphorylation site. In these studies, we demonstrate that the phosphorylation of Y(119) in response to Epo downregulates Jak2 kinase activity. Using a phosphorylation mimic mutation (Y(119)E), downregulation is shown to involve dissociation of Jak2 from the receptor complex. Conversely, a Y(119)F mutant is more stably associated with the receptor complex. Thus, in cytokine responses, ligand binding induces activation of receptor associated Jak2, autophosphorylation of Y(119) in the FERM domain and the subsequent dissociation of the activated Jak2 from the receptor and degradation. This regulation occurs with the receptors for Epo, thrombopoietin and growth hormone but not with the receptor for interferon-gamma.

Abundant hypermethylation of SOCS-1 in clinically silent pituitary adenomas

Janus kinase (JAK)/signal transducers and activators of transcription (STAT) cascade are required for cytokines, growth factors, G-proteins and hormones (growth hormone and prolactin). Gatekeepers in this pathway are the suppressor of cytokine signalling (SOCS) family of proteins. Their expression level is epigenetically regulated by DNA methylation. We have investigated the CpG island methylation status of SOCS-1 in a cohort of pituitary adenomas (PA; n=57), craniopharyngiomas (CP; n=30) and normal pituitary tissue (NP; n=11) using methylation sensitive single-strand conformation polymorphism analysis (MS-SSCP) and direct sequencing. SOCS-1 hypermethylation was identified in 51% (29/57) of surgical specimens obtained from PA patients. 83% of these tumours were clinically silent. In contrast, no methylation of SOCS-1 was observed in CPs or NPs. Quantitative real-time PCR and western blot analysis confirmed reduced SOCS-1 expression in the majority of pituitary adenomas. The data is compatible with epigenetic silencing of the SOCS-1 gene and constitutive activation of the JAK-STAT pathway in PA. This appears to contribute particularly to those tumours characterized by a hormone-inactive status.

Prolactin modulates IL-8 production induced by porins or LPS through different signaling mechanisms

Prolactin (PRL) induces cell proliferation and cell differentiation through the well-known mitogen-activated protein kinases (MAPKs) and Janus kinase (JAK)/signal transducer and activator of transcription (STAT) pathways, depending on the cell line. MAPKs play a central role in signaling transduction mechanisms that transmit mitogenic or differentiation signals from an activated receptor to the intracellular machinery. All of the cytokine receptors that activate the JAK/STAT pathway also activate the MAPK pathway. The aim of the present study was to delineate the signal pathways implicated in IL-8 release by THP-1 cells, pretreated with PRL, after stimulation with either lipopolysaccharide (LPS) or porins from Salmonella enterica serovar Typhimurium. PRL activates the JAK2/STAT1-3 signaling pathway, while LPS or porins from S. enterica serovar Typhimurium does not induce any phosphorylation of this pathway. However, in THP-1 cells, the combination of PRL followed by either S. enterica serovar Typhimurium LPS or porins produced a greater MEK1-MEK2/MAPKs activation response than treatment with PRL alone. Similarly, PRL pretreatment of THP-1 cells resulted in an increase in IL-8 release in response to stimulation with either LPS or porins. This additive effect on IL-8 release was reduced when the cells were also treated with PD-098059, a selective inhibitor of the MEK1 activator and the MAPK cascade, or SB203580, a specific inhibitor of the p38 pathway, or AG490, a specific JAK/STAT pathway inhibitor, providing evidence that there are different signal pathways activated which have a cumulative effect.

Tyrosine 813 is a site of JAK2 autophosphorylation critical for activation of JAK2 by SH2-B beta

The tyrosine kinase Janus kinase 2 (JAK2) binds to the majority of the known members of the cytokine family of receptors. Ligand-receptor binding leads to activation of the associated JAK2 molecules, resulting in rapid autophosphorylation of multiple tyrosines within JAK2. Phosphotyrosines can then serve as docking sites for downstream JAK2 signaling molecules. Despite the importance of these phosphotyrosines in JAK2 function, only a few sites and binding partners have been identified. Using two-dimensional phosphopeptide mapping and a phosphospecific antibody, we identified tyrosine 813 as a site of JAK2 autophosphorylation of overexpressed JAK2 and endogenous JAK2 activated by growth hormone. Tyrosine 813 is contained within a YXXL sequence motif associated with several other identified JAK2 phosphorylation sites. We show that phosphorylation of tyrosine 813 is required for the SH2 domain-containing adapter protein SH2-B beta to bind JAK2 and to enhance the activity of JAK2 and STAT5B. The homologous tyrosine in JAK3, tyrosine 785, is autophosphorylated in response to interleukin-2 stimulation and is required for SH2-B beta to bind JAK3. Taken together these data strongly suggest that tyrosine 813 is a site of autophosphorylation in JAK2 and is the SH2-B beta-binding site within JAK2 that is required for SH2-B beta to enhance activation of JAK2.

SOCS3: an essential regulator of LIF receptor signaling in trophoblast giant cell differentiation

Suppressor of cytokine signaling 3 (SOCS3) binds cytokine receptors and thereby suppresses cytokine signaling. Deletion of SOCS3 causes an embryonic lethality that is rescued by a tetraploid rescue approach, demonstrating an essential role in placental development and a non-essential role in embryo development. Rescued SOCS3-deficient mice show a perinatal lethality with cardiac hypertrophy. SOCS3-deficient placentas have reduced spongiotrophoblasts and increased trophoblast secondary giant cells. Enforced expression of SOCS3 in a trophoblast stem cell line (Rcho-1) suppresses giant cell differentiation. Conversely, SOCS3-deficient trophoblast stem cells differentiate more readily to giant cells in culture, demonstrating that SOCS3 negatively regulates trophoblast giant cell differentiation. Leukemia inhibitory factor (LIF) promotes giant cell differentiation in vitro, and LIF receptor (LIFR) deficiency results in loss of giant cell differentiation in vivo. Finally, LIFR deficiency rescues the SOCS3-deficient placental defect and embryonic lethality. The results establish SOCS3 as an essential regulator of LIFR signaling in trophoblast differentiation.

Essential, nonredundant role for the phosphoinositide 3-kinase p110delta in signaling by the B-cell receptor complex

Many receptor and nonreceptor tyrosine kinases activate phosphoinositide 3-kinases (PI3Ks). To assess the role of the delta isoform of the p110 catalytic subunit of PI3Ks, we derived enzyme-deficient mice. The mice are viable but have decreased numbers of mature B cells, a block in pro-B-cell differentiation, and a B1 B-cell deficiency. Both immunoglobulin M receptor-induced Ca(2+) flux and proliferation in response to B-cell mitogens are attenuated. Immunoglobulin levels are decreased substantially. The ability to respond to T-cell-independent antigens is markedly reduced, and the ability to respond to T-cell-dependent antigens is completely eliminated. Germinal center formation in the spleen in response to antigen stimulation is disrupted. These results define a nonredundant signaling pathway(s) utilizing the delta isoform of p110 PI3K for the development and function of B cells.

Identification, cDNA cloning, and targeted deletion of p70, a novel, ubiquitously expressed SH3 domain-containing protein

In a screen for proteins that interact with Jak2, we identified a previously uncharacterized 70-kDa protein and cloned the corresponding cDNA. The predicated sequence indicates that p70 contains an SH3 domain and a C-terminal domain with similarities to the catalytic motif of phosphoglycerate mutase. p70 transcripts were found in all tissues examined. Similarly, when an antibody raised against a C-terminal peptide to analyze p70 protein expression was used, all murine tissues examined were found to express p70. To investigate the in vivo role of p70, we generated a p70-deficient mouse strain. Mice lacking p70 are viable, develop normally, and do not display any obvious abnormalities. No differences were detected in various hematological parameters, including bone marrow colony-forming ability, in response to cytokines that utilize Jak2. In addition, no impairment in B- and T-cell development and proliferative ability was detected.

Role of STAT6 in IgE receptor/FcepsilonRI-mediated late phase allergic responses of mast cells

In this study we show that IgE receptor engagement triggers activation of STAT6 in mast cells. We sought to determine the role of STAT6 activation in IgE receptor-mediated mast cell responses using STAT6 knockout mice. After IgE receptor engagement, bone marrow mast cells from STAT6(-/-) mice exhibited normal histamine and leukotriene C(4) release, but their cytokine release was markedly reduced. In accordance with these in vitro data, IgE/Ag-challenged STAT6(-/-) mice showed normal early phase, but severely impaired late phase, allergic reactions. These findings provide unprecedented evidence that STAT6 plays a pivotal role in mast cell responses to IgE/Ag stimulation.

Cytokine signalling and disease

Cytokines mediate their response via cell surface receptors that in turn activate intracellular signalling pathways and lead to gene activation, cell proliferation and differentiation. Many recent studies have shown that cytokine and cytokine receptor pathways are frequently mutated in disease, thus shedding light on the generation of the inflammatory response, specific immunity and mechanisms of haematopoiesis. Many approaches are being used to translate this basic research into successful therapies and although host immune responses involve many different cells and crucial pathways, modulation of therapeutic responses can be induced or inhibited by, targeting a single cytokine. This review summarises current knowledge of cytokine pathways in disease and the use of cytokine- or receptor-directed therapy to exploit the immune response to disease.

Constitutive activation of STAT-3 and downregulation of SOCS-3 expression induced by adrenalectomy

Removal of adrenal steroids by adrenalectomy (ADX) slows or reverses the development of many forms of obesity in rodents, including those that are leptin or leptin receptor deficient. Obesity is associated with hyperleptinemia and leptin resistance. We hypothesized that glucocorticoids impair leptin receptor signaling and that removal thereof would activate the Janus kinase (JAK)-signal transducers and activators of transcription (STAT) signaling pathway. The inhibitory effect of leptin (2.5 microg icv) on food intake was enhanced in ADX rats. A combination of ribonuclease protection assays, RT-PCR, Western blots, and mobility shift assays was used to evaluate the leptin signaling pathway in whole hypothalami from sham-operated, ADX and corticosterone-replaced ADX (ADX-R) Sprague-Dawley rats that were treated acutely with either saline vehicle or leptin intracerebroventricularly. ADX increased the expression of leptin receptor mRNA, increased STAT-3 mRNA and protein levels, induced constitutive STAT-3 phosphorylation and DNA binding activity, and also reduced suppressor of cytokine signaling-3 (SOCS-3) mRNA and protein levels. ADX and leptin treatment increased STAT-3 phosphorylation, but with no concomitant increase in DNA binding activity. Leptin and ADX decreased NPY mRNA expression, but their combination did not further decrease NPY mRNA. Corticosterone supplementation of ADX rats partially reversed many of these effects. In conclusion, ADX through activation of STAT-3 and inhibition of SOCS-3 activates the JAK-STAT signaling pathway. These effects most probably explain the ability to prevent the development of obesity by removal of adrenal steroids.

Dysfunction of Stat4 leads to accelerated incidence of chemical-induced thymic lymphomas in mice

Stat4 (signal transducer and activator of transcription) can be activated by specific cytokines, such as IL-12, IFN-alpha, and IL-2. Since IL-12 has been implicated in tumor surveillance and cancer treatment, we hypothesized that its signaling mediator, Stat4, may repress tumor growth. Mice lacking Stat4 allowed us to directly assess the role of Stat4 in tumor surveillance. Lymphomas were chemically induced by MNU (N-methyl-N-nitrosourea) injection in Stat4-deficient or wild-type control mice. At the time of termination of the experiment 16 weeks after injection, 78% of homozygous Stat4-deficient mice had developed thymic lymphomas. This tumor induction was dramatically higher than in heterozygous (14%) and wild-type controls (14%). Lymphoma development occurred 5 weeks earlier in homozygous knockout mice than in other genotypes. Mice bearing tumors were fragile and had an increased death rate in the early stages of the experiment. The tumors displayed a very aggressive phenotype with metastases in multiple organs. Therefore, the loss of Stat4 predisposes mice to tumor induction and demonstrates crucial roles of Stat4 in the prevention of tumors.

Differentiation therapy of human cancer: basic science and clinical applications

Current cancer therapies are highly toxic and often nonspecific. A potentially less toxic approach to treating this prevalent disease employs agents that modify cancer cell differentiation, termed 'differentiation therapy.' This approach is based on the tacit assumption that many neoplastic cell types exhibit reversible defects in differentiation, which upon appropriate treatment, results in tumor reprogramming and a concomitant loss in proliferative capacity and induction of terminal differentiation or apoptosis (programmed cell death). Laboratory studies that focus on elucidating mechanisms of action are demonstrating the effectiveness of 'differentiation therapy,' which is now beginning to show translational promise in the clinical setting.

Differential binding to and regulation of JAK2 by the SH2 domain and N-terminal region of SH2-bbeta

SH2-Bbeta has been shown to bind via its SH2 (Src homology 2) domain to tyrosyl-phosphorylated JAK2 and strongly activate JAK2. In this study, we demonstrate the existence of an additional binding site(s) for JAK2 within the N-terminal region of SH2-Bbeta (amino acids 1 to 555) and the ability of this region of SH2-B to inhibit JAK2. Four lines of evidence support the existence of this additional binding site(s). In a glutathione S-transferase pull-down assay, wild-type SH2-Bbeta and SH2-Bbeta(R555E) with a defective SH2 domain bind to both tyrosyl-phosphorylated JAK2 from growth hormone (GH)-treated cells and non-tyrosyl-phosphorylated JAK2 from control cells, whereas the SH2 domain of SH2-Bbeta binds only to tyrosyl-phosphorylated JAK2 from GH-treated cells. Similarly, JAK2 is present in alphaSH2-B immunoprecipitates in the absence and presence of GH, with GH substantially increasing the coprecipitation of JAK2 with SH2-B. When coexpressed in COS cells, SH2-Bbeta coimmunoprecipitates not only wild-type, tyrosyl-phosphorylated JAK2 but also kinase-inactive, non-tyrosyl-phosphorylated JAK2(K882E), although to a lesser extent. DeltaC555 (amino acids 1 to 555 of SH2-Bbeta) that lacks most of the SH2 domain binds similarly to wild-type JAK2 and kinase-inactive JAK2(K882E). Experiments using a series of N- and C-terminally truncated SH2-Bbeta constructs indicate that the pleckstrin homology (PH) domain (amino acids 269 to 410) and amino acids 410 to 555 are necessary for maximal binding of SH2-Bbeta to inactive JAK2, but neither region alone is sufficient for maximal binding. The SH2 domain of SH2-Bbeta is necessary and sufficient for the stimulatory effect of SH2-Bbeta on JAK2 and JAK2-mediated tyrosyl phosphorylation of Stat5B. In contrast, DeltaC555 lacking the SH2 domain, and to a lesser extent the PH domain alone, inhibits JAK2. DeltaC555 also blocks JAK2-mediated tyrosyl phosphorylation of Stat5B in COS cells and GH-stimulated nuclear accumulation of Stat5B in 3T3-F442A cells. These data indicate that in addition to the SH2 domain, SH2-Bbeta has one or more lower-affinity binding sites for JAK2 within amino acids 269 to 555. The interaction via this site(s) in SH2-B with inactive JAK2 seems likely to increase the local concentration of SH2-Bbeta around JAK2, thereby facilitating binding of the SH2 domain to ligand-activated JAK2. This would result in a more rapid and robust cellular response to hormones and cytokines that activate JAK2. This interaction between inactive JAK2 and SH2-B may also help prevent abnormal activation of JAK2.

Mimicry of erythropoietin by a nonpeptide molecule

Erythropoietin (EPO) controls the proliferation and differentiation of erythroid progenitor cells into red blood cells. EPO induces these effects by dimerization of the EPO receptors (EPOR) present on these cells. To discover nonpeptide molecules capable of mimicking the effects of EPO, we identified a small molecule capable of binding to one chain of EPOR and used it to synthesize molecules capable of inducing dimerization of the EPOR. We first identified compound 1 (N-3-[2-(4-biphenyl)-6-chloro-5-methyl]indolyl-acetyl-L-lysine methyl ester) by screening the in-house chemical collection for inhibitors of EPO binding to human EPOR and then prepared compound 5, which contains eight copies of compound 1 held together by a central core. Although both compounds inhibited EPO binding of EPOR, only compound 5 induced dimerization of soluble EPOR. Binding of EPO to its receptor in cells results in activation of many intracellular signaling molecules, including transcription factors like signal transducer and activator of transcription (STAT) proteins, leading to growth and differentiation of these cells. Consistent with its ability to induce dimerization of EPOR in solution, compound 5 exhibited much of the same biological activities as EPO, such as (i) the activation of a STAT-dependent luciferase reporter gene in BAF3 cells expressing human EPOR, (ii) supporting the proliferation of several tumor cell lines expressing the human or mouse EPOR, and (iii) the in vitro differentiation of human progenitor cells into colonies of erythrocytic lineage. These data demonstrate that a nonpeptide molecule is capable of inducing EPOR dimerization and mimicking the biological activities of EPO.

Targeting Janus kinase 3 in mast cells prevents immediate hypersensitivity reactions and anaphylaxis

Janus kinase 3 (JAK3), a member of the Janus family protein-tyrosine kinases, is expressed in mast cells, and its enzymatic activity is enhanced by IgE receptor/FcepsilonRI cross-linking. Selective inhibition of JAK3 in mast cells with 4-(4'-hydroxylphenyl)-amino-6, 7-dimethoxyquinazoline) (WHI-P131) blocked the phospholipase C activation, calcium mobilization, and activation of microtubule-associated protein kinase after lgE receptor/FcepsilonRI cross-linking. Treatment of IgE-sensitized rodent as well as human mast cells with WHI-P131 effectively inhibited the activation-associated morphological changes, degranulation, and proinflammatory mediator release after specific antigen challenge without affecting the functional integrity of the distal secretory machinery. In vivo administration of the JAK3 inhibitor WHI-P131 prevented mast cell degranulation and development of cutaneous as well as systemic fatal anaphylaxis in mice at nontoxic dose levels. Thus, JAK3 plays a pivotal role in IgE receptor/FcepsilonRI-mediated mast cell responses, and targeting JAK3 with a specific inhibitor, such as WHI-P131, may provide the basis for new and effective treatment as well as prevention programs for mast cell-mediated allergic reactions.

Identification of SH2-bbeta as a potent cytoplasmic activator of the tyrosine kinase Janus kinase 2

Janus kinases (JAKs) are cytoplasmic tyrosine kinases critical for signaling by growth hormone (GH) and many other ligands that bind to members of the cytokine receptor superfamily. SH2-Bbeta was previously identified as a JAK2-interacting protein that is tyrosyl phosphorylated in response to GH and other cytokines that activate JAK2. In this study, we examined whether SH2-Bbeta alters the activity of JAK2. SH2-Bbeta, when coexpressed with JAK2, significantly increased the tyrosyl phosphorylation of JAK2 and multiple other cellular proteins and stimulated the kinase activity of JAK2 by approximately 20-fold. Coexpression of SH2-Bbeta with JAK2 dramatically increased tyrosyl phosphorylation of signal transducer and activator of transcription (Stat)5B and Stat3, physiological substrates of JAK2. SH2-Bbeta(R555E) with a defective Src homology 2 domain was unable to stimulate JAK2 and JAK2-mediated tyrosyl phosphorylation of Stat5B and Stat3. More importantly, SH2-Bbeta enhanced GH-induced tyrosyl phosphorylation of endogenous JAK2 and ligand-induced tyrosyl phosphorylation of Stat5B by endogenous JAK2. In contrast, SH2-Bbeta did not potentiate the activation of other tyrosine kinases including the receptors for platelet-derived growth factor, epidermal growth factor, or nerve growth factor (TrkA), tyrosine kinases that also bind SH2-Bbeta. These data demonstrate that SH2-Bbeta is a potent cytoplasmic activator of JAK2 and is thereby expected to be an important cellular regulator of signaling by GH and other hormones and cytokines that activate JAK2.

Signaling by proinflammatory cytokines: oligomerization of TRAF2 and TRAF6 is sufficient for JNK and IKK activation and target gene induction via an amino-terminal effector domain

Interleukin-1 (IL-1) and tumor necrosis factor (TNF-alpha) stimulate transcription factors AP-1 and NF-kappaB through activation of the MAP kinases JNK and p38 and the IkappaB kinase (IKK), respectively. The TNF-alpha and IL-1 signals are transduced through TRAF2 and TRAF6, respectively. Overexpressed TRAF2 or TRAF6 activate JNK, p38, or IKK in the absence of extracellular stimulation. By replacing the carboxy-terminal TRAF domain of TRAF2 and TRAF6 with repeats of the immunophilin FKBP12, we demonstrate that their effector domains are composed of their amino-terminal Zn and RING fingers. Oligomerization of the TRAF2 effector domain results in specific binding to MEKK1, a protein kinase capable of JNK, p38, and IKK activation, and induction of TNF-alpha and IL-1 responsive genes. TNF-alpha also enhances the binding of native TRAF2 to MEKK1 and stimulates the kinase activity of the latter. Thus, TNF-alpha and IL-1 signaling is based on oligomerization of TRAF2 and TRAF6 leading to activation of effector kinases.

Thrombopoietin signal transduction requires functional JAK2, not TYK2

The Janus family of tyrosine kinases (JAKs) plays a critical role in signal transduction by members of the cytokine receptor superfamily. In response to ligand-receptor interaction, these nonreceptor tyrosine kinases are rapidly phosphorylated and activated, triggering tyrosine phosphorylation and activation of downstream signaling intermediates. Upon binding to its receptor, the product of the proto-oncogene c-mpl, thrombopoietin (TPO) activates both JAK2 and TYK2 in multiple cell lines as well as megakaryocytes and platelets. To study whether one or both of these kinases are essential for TPO signal transduction, we engineered a parental human sarcoma cell line (2C4) as well as sarcoma cell lines that are deficient in JAK2 expression (gamma2A) or TYK2 expression (U1A) to express the wild-type Mpl receptor. The ability of TPO to induce tyrosine phosphorylation of Mpl and multiple intracellular substrates in each cell line was then examined. Our results demonstrate that JAK2-deficient cells (gamma2A-Mpl) are unable to initiate TPO-mediated signaling. In contrast, cells that are TYK2-deficient (U1A-Mpl) are able to induce tyrosine phosphorylation of Mpl, JAK2, STAT3, and Shc as efficiently as parental cells (2C4-Mpl). These data indicate that JAK2 is an essential component of Mpl signaling and that, in the absence of JAK2, TYK2 is incapable of initiating TPO-induced tyrosine phosphorylation.

Granulocyte-macrophage colony-stimulating factor-activated signaling pathways in human neutrophils. Involvement of Jak2 in the stimulation of phosphatidylinositol 3-kinase

Granulocyte-macrophage colony-stimulating factor (GM-CSF) regulates many of the biological activities of human neutrophils. The signaling pathways via which these effects are mediated are not fully understood. We have shown previously that GM-CSF treatment of human neutrophils activates the Janus kinase/signal transducers and activators of transcription (Jak/STAT) pathway and, more specifically, Jak2, STAT3, and STAT5B in neutrophils. GM-CSF also stimulates the activity of the phosphatidylinositol 3-kinase (PI3-kinase) in a tyrosine kinase-dependent manner. Here we report that pretreating the cells with a Jak2 inhibitor (AG-490) abolishes tyrosine phosphorylation of the p85 subunit of PI3-kinase induced by GM-CSF. Furthermore, p85 was found to associate with Jak2, but not with Lyn, in stimulated cells in situ and with its autophosphorylated form in vitro; however, Jak2 did not bind to either of the two Src homology 2 (SH2) domains of the p85 subunit of PI3-kinase. Although STAT5B bound to the carboxyl-terminal SH2 domain of p85, it was absent from the complex containing PI3-kinase and Jak2. These results suggest that stimulation of the activity of PI3-kinase induced by GM-CSF is mediated by Jak2 and that the association between Jak2 and p85 depends on an adaptor protein yet to be identified.

Virus-specific immunity after gene therapy in a murine model of severe combined immunodeficiency

Human severe combined immunodeficiency (SCID) can be caused by defects in Janus kinase 3 (JAK3)-dependent cytokine signaling pathways. As a result, patients are at high risk of life-threatening infection. A JAK3 -/- SCID mouse model for the human disease has been used to test whether transplant with retrovirally transduced bone marrow (BM) cells (JAK3 BMT) could restore immunity to an influenza A virus. The immune responses also were compared directly with those for mice transplanted with wild-type BM (+/+ BMT). After infection, approximately 90% of the JAK3 BMT or +/+ BMT mice survived, whereas all of the JAK3 -/- mice died within 29 days. Normal levels of influenza-specific IgG were present in plasma from JAK3 BMT mice at 14 days after respiratory challenge, indicating restoration of B cell function. Influenza-specific CD4(+) and CD8(+) T cells were detected in the spleen and lymph nodes, and virus-specific CD8(+) effectors localized to the lungs of the JAK3 BMT mice. The kinetics of the specific host response correlated with complete clearance of the virus within 2 weeks of the initial exposure. By contrast, the JAK3 -/- mice did not show any evidence of viral immunity and were unable to control this viral pneumonia. Retroviral-mediated JAK3 gene transfer thus restores diverse aspects of cellular and humoral immunity and has obvious potential for human autologous BMT.

Negative regulation of mitosis in fission yeast by the shk1 interacting protein skb1 and its human homolog, Skb1Hs

We previously provided evidence that the protein encoded by the highly conserved skb1 gene is a putative regulator of Shk1, a p21(Cdc42/Rac)-activated kinase (PAK) homolog in the fission yeast Schizosaccharomyces pombe. skb1 null mutants are viable and competent for mating but less elongate than wild-type S. pombe cells, whereas cells that overexpress skb1 are hyperelongated. These phenotypes suggest a possible role for Skb1 as a mitotic inhibitor. Here we show genetic interactions of both skb1 and shk1 with genes encoding key mitotic regulators in S. pombe. Our results indicate that Skb1 negatively regulates mitosis by a mechanism that is independent of the Cdc2-activating phosphatase Cdc25 but that is at least partially dependent on Shk1 and the Cdc2 inhibitory kinase Wee1. We provide biochemical evidence for association of Skb1 and Shk1 with Cdc2 in S. pombe, suggesting that Skb1 and Shk1 inhibit mitosis through interaction with the Cdc2 complex, rather than by an indirect mechanism. These results provide evidence of a previously undescribed role for PAK-related protein kinases as mitotic inhibitors. We also describe the cloning of a human homolog of skb1, SKB1Hs, and show that it can functionally replace skb1 in S. pombe. Thus, the molecular functions of Skb1-related proteins have likely been substantially conserved through evolution.

Tumor necrosis factor-alpha mediated activation of signal transduction cascades and transcription factors in 3T3-L1 adipocytes

We have previously demonstrated that exposure of fully differentiated 3T3-L1 adipocytes to TNF results in an activation of at least two separate signal transduction pathways: 1. the sphingomyelinase leading to generation of ceramide and 2. the proliferative and cell growth regulating p44/42 MAP kinase cascade. In the current study we extend those observations and examine the ability of both TNF and ceramide to activate the stress/cytokine activated p38 MAPK, the JNK and JAK-STAT pathways. Interestingly, the p38 MAP kinase was observed to be constitutively active and its phosphorylation status (activation) was not altered with either TNF or ceramide treatment. Analysis of the JNK and JAK-STAT pathways also demonstrated an absence of TNF-induced activation. Similar results were obtained when the adipocytes were treated with a cell permeable analog of ceramide. However, the adipocytes were observed to respond to TNF with a rapid alteration in the GSH-GSSG equilibrium in a manner consistent with a cellular response to an oxidative stress. This response may mediate the TNF-induced metabolic disturbances observed in the adipose cell.

Biology of the interleukin-2 receptor

Studies of the biology of the IL-2 receptor have played a major part in establishing several of the fundamental principles that govern our current understanding of immunology. Chief among these is the contribution made by lymphokines to regulation of the interactions among vast numbers of lymphocytes, comprising a number of functionally distinct lineages. These soluble mediators likely act locally, within the context of the microanatomic organization of the primary and secondary lymphoid organs, where, in combination with signals generated by direct membrane-membrane interactions, a wide spectrum of cell fate decisions is influenced. The properties of IL-2 as a T-cell growth factor spawned the view that IL-2 worked in vivo to promote clonal T-cell expansion during immune responses. Over time, this singular view has suffered from increasing appreciation that the biologic effects of IL-2R signals are much more complex than simply mediating T-cell growth: depending on the set of conditions, IL-2R signals may also promote cell survival, effector function, and apoptosis. These sometimes contradictory effects underscore the fact that a diversity of intracellular signaling pathways are potentially activated by IL-2R. Furthermore, cell fate decisions are based on the integration of multiple signals received by a lymphocyte from the environment; IL-2R signals can thus be regarded as one input to this integration process. In part because IL-2 was first identified as a T-cell growth factor, the major focus of investigation in IL-R2 signaling has been on the mechanism of mitogenic effects in cultured cell lines. Three critical events have been identified in the generation of the IL-2R signal for cell cycle progression, including heterodimerization of the cytoplasmic domains of the IL-2R beta and gamma(c) chains, activation of the tyrosine kinase Jak3, and phosphorylation of tyrosine residues on the IL-2R beta chain. These proximal events led to the creation of an activated receptor complex, to which various cytoplasmic signaling molecules are recruited and become substrates for regulatory enzymes (especially tyrosine kinases) that are associated with the receptor. One intriguing outcome of the IL-2R signaling studies performed in cell lines is the apparent functional redundancy of the A and H regions of IL-2R beta, and their corresponding downstream pathways, with respect to the proliferative response. Why should the receptor complex induce cell proliferation through more than one mechanism or pathway? One possibility is that this redundancy is an unusual property of cultured cell lines and that primary lymphocytes require signals from both the A and the H regions of IL-2R beta for optimal proliferative responses in vivo. An alternative possibility is that the A and H regions of IL-2R beta are only redundant with respect to proliferation and that each region plays a unique and essential role in regulating other aspects of lymphocyte physiology. As examples, the A or H region could prove to be important for regulating the sensitivity of lymphocytes to AICD or for promoting the development of NK cells. These issues may be resolved by reconstituting IL-2R beta-/-mice with A-and H-deleted forms of the receptor chain and analyzing the effect on lymphocyte development and function in vivo. In addition to the redundant nature of the A and H regions, there remains a large number of biochemical activities mediated by the IL-2R for which no clear physiological role has been identified. Therefore, the circumstances are ripe for discovering new connections between molecular signaling events activated by the IL-2R and the regulation of immune physiology. Translating biochemical studies of Il-2R function into an understanding of how these signals regulate the immune system has been facilitated by the identification of natural mutations in IL-2R components in humans with immunodeficiency and by the generation of mice with targeted mutations in these gen

Bruton's tyrosine kinase (BTK) as a dual-function regulator of apoptosis

Multiple counterregulatory mechanisms have been identified in B-cell precursors that operate to regulate cell survival and growth, thereby ensuring the orderly development and differentiation of B-cells. Inappropriate apoptosis may underlie the pathogenesis of immunodeficiencies, as well as pathogenesis and drug/radiation resistance of human leukemias and lymphomas, which makes control of apoptosis an important potential target for therapeutic interventions. Therefore, identification of the molecular regulators of apoptosis is an area of intense investigation. Bruton's tyrosine kinase (BTK) is the first tyrosine kinase to be identified as a dual-function regulator of apoptosis, which promotes radiation-induced apoptosis but inhibits Fas-activated apoptosis in B-cells. BTK functions in a pro-apoptotic manner when B-cells are exposed to reactive oxygen intermediates, at least in part, by down-regulating the anti-apoptotic activity of STAT-3 transcription factor. In contrast, BTK associates with the death receptor Fas and impairs its interaction with Fas-associated protein with death domain (FADD), which is essential for the recruitment and activation of FLICE by Fas during the apoptotic signal, thereby preventing the assembly of a pro-apoptotic death inducing signaling complex (DISC) after Fas-ligation. The identification of BTK as a dual-function regulator of apoptosis will significantly increase our understanding of both the biological processes involved in programmed cell death and the diseases associated with dysregulation of apoptosis. New agents with BTK-modulatory activity may have clinical potential in the treatment of B-cell malignancies (in particular acute lymphoblastic leukemia, the most common form of childhood cancer), as well as B-cell immunodeficiencies.

Erythropoietin Induces Tyrosine Phosphorylation of Jak2, STAT5A, and STAT5B in Primary Cultured Human Erythroid Precursors

We examined signaling by erythropoietin in highly purified human colony forming unit-erythroid cells, generated in vitro from CD34+ cells. We found that erythropoietin induces tyrosine phosphorylation of Jak2, STAT5A, and STAT5B. Tyrosine phosphorylation of Jak2 reaches a peak around 10 minutes after stimulation and is maximum at 5 U/mL of erythropoietin. Tyrosine phosphorylation of STAT5 is accompanied by the translocation of activated STAT5 to the nucleus as shown by electrophoretic mobility shift assay (EMSA) using 32Pi-labeled STAT5 binding site in the β-casein promoter. Tyrosine phosphorylation STAT1 or STAT3 was not detected in human erythroid precursors after stimulation with erythropoietin. Crkl, an SH2/SH3 adapter protein, becomes coimmunoprecipitated specifically with STAT5 from erythropoietin-stimulated erythroid cells; although it was shown to become associated with c-Cbl in the studies using cell lines. Thus, human erythroid precursors can be expanded in vitro in sufficient numbers and purity to allow its usage in signal transduction studies. This report sets a basis for further studies on signaling in primary cultured human erythroid precursors, which in turn contribute to our better understanding in the differentiation processes of erythrocytes and their precursors.

Erythropoietin Induces Tyrosine Phosphorylation of Jak2, STAT5A, and STAT5B in Primary Cultured Human Erythroid Precursors

We examined signaling by erythropoietin in highly purified human colony forming unit-erythroid cells, generated in vitro from CD34+ cells. We found that erythropoietin induces tyrosine phosphorylation of Jak2, STAT5A, and STAT5B. Tyrosine phosphorylation of Jak2 reaches a peak around 10 minutes after stimulation and is maximum at 5 U/mL of erythropoietin. Tyrosine phosphorylation of STAT5 is accompanied by the translocation of activated STAT5 to the nucleus as shown by electrophoretic mobility shift assay (EMSA) using 32Pi-labeled STAT5 binding site in the β-casein promoter. Tyrosine phosphorylation STAT1 or STAT3 was not detected in human erythroid precursors after stimulation with erythropoietin. Crkl, an SH2/SH3 adapter protein, becomes coimmunoprecipitated specifically with STAT5 from erythropoietin-stimulated erythroid cells; although it was shown to become associated with c-Cbl in the studies using cell lines. Thus, human erythroid precursors can be expanded in vitro in sufficient numbers and purity to allow its usage in signal transduction studies. This report sets a basis for further studies on signaling in primary cultured human erythroid precursors, which in turn contribute to our better understanding in the differentiation processes of erythrocytes and their precursors.