JAK2 Kinase Activity Research Articles

Cytokine receptor signal transduction through Jak tyrosine kinases and Stat transcription factors.

Cytokines are the principal regulators of cell proliferation and differentiation of hematopoietic cells and these responses are initiated through activation of hematopoietic cytokine receptors. Although the receptor intracellular domains lack any kinase domains, activation of cytokine receptors lead to rapid induction of tyrosine phosphorylation. Recently, cytokine receptors have been shown to associate with and activate members of the cytoplasmic Jak tyrosine kinase family. Activation of Jak kinases leads to phosphorylation of several signaling proteins and thereby couples ligand-mediated receptor stimulation to activation of intracellular signaling pathways. The best characterized substrates for Jaks are the Stat transcription factors, which are crucial mediators of cytokine-mediated gene responses, and, particularly, central determinants for the specificity in cytokine responses.

The role of GH receptor tyrosine phosphorylation in Stat5 activation

Stimulation of GH receptors leads to rapid activation of Jak2 kinase and subsequent tyrosine phosphorylation of the GH receptor. Three specific tyrosines located in the C-terminal domain of the GH receptor have been identified as being involved in GH-stimulated transcription of the Spi 2.1 promoter. Mutated GH receptors lacking all but one of these three tyrosines are able to mediate a transcriptional response when transiently transfected into CHO cells together with a Spi 2.1 promoter/luciferase construct. Similarly, these GH receptors were found to be able to mediate activation of Stat5 DNA-binding activity, whereas the GH receptor mutant lacking all intracellular tyrosines was not. Synthetic tyrosine phosphorylated peptides corresponding to the GH receptor sequence around the three tyrosines inhibited Stat5 DNA-binding activity while their non-phosphorylated counterparts were ineffective. Tyrosine phosphorylated GST-GH receptor fusion proteins specifically bound to Stat5 in extracts from COS 7 cells transfected with Stat5 cDNA. This binding could be inhibited by tyrosine phosphorylated peptides derived from the GH receptor. This study thus demonstrated that specific GH receptor tyrosine residues, in their phosphorylated state, are involved in transcriptional signaling by directly interacting with Stat5.

Signal transduction of granulocyte macrophage-colony stimulating factor in a human endothelium-derived cell line.

Granulocyte macrophage-colony stimulating factor (GM-CSF) regulates the growth and differentiation of hematopoietic cells and is also involved in angiogenesis. The induction of protein tyrosine phosphorylation is critical for cytokines and growth factor-mediated signal transduction. The protein tyrosine kinase (PTK), JAK2 is involved in signaling through a number of cytokine receptors, including GM-CSF receptors. In the present study, we investigated the effect of GM-CSF on the cell cycle and protein tyrosine phosphorylation in a human endothelial cell-derived cell line, EA.hy 926 cells. GM-CSF induced the cell cycle progression and tyrosine phosphorylation of cellular proteins including JAK2 kinase in EA.hy 926 cells. Herbimycin A, a PTK inhibitor, completely blocked the GM-CSF-induced cell cycle progression, protein tyrosine phosphorylation and JAK2 kinase activation in EA.hy 926 cells. Our results demonstrate that protein tyrosine phosphorylation and JAK2 kinase activation are closely related to the GM-CSF-mediated signal transduction and growth in vascular endothelial cells, and suggest the efficacy of herbimycin A in controlling angiogenesis.

Growth Hormone, Interferon-γ, and Leukemia Inhibitory Factor Utilize Insulin Receptor Substrate-2 in Intracellular Signaling

In this report, we demonstrate that insulin receptor substrate-2 (IRS-2) is tyrosyl-phosphorylated following stimulation of 3T3-F442A fibroblasts with growth hormone (GH), leukemia inhibitory factor and interferon-gamma. In response to GH and leukemia inhibitory factor, IRS-2 is immediately phosphorylated, with maximal phosphorylation detected at 15 min; the signal is substantially diminished by 60 min. In response to interferon-gamma, tyrosine phosphorylation of IRS-2 was prolonged, with substantial signal still detected at 60 min. Characterization of the mechanism of signaling utilized by GH indicated that tyrosine residues in GH receptor are not necessary for tyrosyl phosphorylation of IRS-2; however, the regions of GH receptor necessary for IRS-2 tyrosyl phosphorylation are the same as those required for JAK2 association and tyrosyl phosphorylation. The role of IRS-2 as a signaling molecule for GH is further demonstrated by the finding that GH stimulates association of IRS-2 with the 85-kDa regulatory subunit of phosphatidylinositol 3'-kinase and with the protein-tyrosine phosphatase SHP2. These results are consistent with the possibility that IRS-2 is a downstream signaling partner of multiple members of the cytokine family of receptors that activate JAK kinases.

Inhibition of cytokines and JAK-STAT activation by distinct signaling pathways.

An important component of cytokine regulation of cell growth and differentiation is rapid transcriptional activation of genes by the JAK-STAT (signal transducer and activator of transcription) signaling pathway. Ligation of cytokine receptors results in tyrosine phosphorylation and activation of receptor-associated Jak protein tyrosine kinases and cytoplasmic STAT transcription factors, which then translocate to the nucleus. We describe the interruption of cytokine triggered JAK-STAT signals by cAMP, the calcium ionophore ionomycin, and granulocyte/macrophage colony-stimulating factor. Jak1 kinase activity, interleukin 6-induced gene activation, Stat3 tyrosine phosphorylation, and DNA-binding were inhibited, as was activation of Jak1 and Stat1 by interferon gamma. The kinetics and requirement for new RNA and protein synthesis for inhibition of interleukin 6 by ionomycin and GM-CSF differed, but both agents increased the association of Jak1 with protein tyrosine phosphatase ID (SH2-containing phosphatase 2). Our results demonstrate that crosstalk with distinct signaling pathways can inhibit JAK-STAT signal transduction, and suggest approaches for modulating cytokine activity during immune responses and inflammatory processes.

Modulation of the expression of the IFN-gamma receptor beta-chain controls responsiveness to IFN-gamma in human peripheral blood T cells.

IFN-gamma has potent antiproliferative and apoptotic effects in T cells that are important in determining T cell development and polarized differentiation. Therefore, any event that enables T cells to become less responsive to IFN- gamma may potentially alter immune responsiveness to Ag. In this work, we show that human peripheral blood T cells that are stimulated through the TCR and expanded with IL-2 are unresponsive to IFN-gamma, as determined by a lack of activation of jak kinases and the transcription factor, STAT1(alpha), a signal transducer and activator of transcription. This nonresponsiveness occurs because of a lack of expression of the beta- chain (accessory factor) of the IFN-gamma receptor, while at the same time maintaining IFN-gamma receptor alpha-chain expression. Expression of the beta-chain can be restored by secondary TCR ligation or PMA treatment. T cell blasts treated with PMA are now responsive to IFN-gamma. When freshly isolated, highly enriched (>98%) T cells are examined for IFN-gamma responsiveness; these cells can respond to IFN-gamma and express beta-chain. Therefore, as T cells progress from primary TCR activation through IL-2-dependent proliferation, followed by secondary TCR stimulation, their responsiveness to IFN-gamma varies, and this may affect their ability to participate in an ongoing immune response.

Identification of Tyrosine Residues in the Intracellular Domain of the Growth Hormone Receptor Required for Transcriptional Signaling and Stat5 Activation

The binding of growth hormone (GH) to its receptor results in its dimerization followed by activation of Jak2 kinase and tyrosine phosphorylation of the GH receptor itself, as well as Jak2 and the transcription factors Stat1, -3, and -5. In order to study the role of GH receptor tyrosine phosphorylation in intracellular signaling, we constructed GH receptors in which combinations of tyrosines were mutated to phenylalanines. We identified three tyrosine residues at positions 534, 566, and 627 that were required for activation of GH-stimulated transcription of the serine protease inhibitor (Spi) 2.1 promoter. Any of these three tyrosines is able to independently mediate GH-induced transcription, indicating redundancy in this part of the GH receptor. Tyrosine phosphorylation was not required for GH stimulation of mitogen-activated protein (MAP) kinase activity or for GH-stimulated Ca2+ channel activation since these pathways were normal in cells expressing a GH receptor in which all eight intracellular tyrosines were mutated to phenylalanines. Activation of Stat5 by GH was, however, abolished in cells expressing the GH receptor lacking intracellular tyrosines. This study demonstrates that specific tyrosines in the GH receptor are required for transcriptional signaling possibly by their role in the activation of transcription factor Stat5.

A membrane-proximal region of the interleukin-2 receptor gamma c chain sufficient for Jak kinase activation and induction of proliferation in T cells.

The interleukin-2 (IL-2) receptor (IL-2R) consists of three distinct subunits (alpha, beta, and gamma c) and regulates proliferation of T lymphocytes. Intracellular signalling results from ligand-mediated heterodimerization of the cytoplasmic domains of the beta and gamma c chains. To identify the residues of gamma c critical to this process, mutations were introduced into the cytoplasmic domain, and the effects on signalling were analyzed in the IL-2-dependent T-cell line CTLL2 and T-helper clone D10, using chimeric IL-2R chains that bind and are activated by granulocyte-macrophage colony-stimulating factor. Whereas previous studies of fibroblasts and transformed T cells have suggested that signalling by gamma c requires both membrane-proximal and C-terminal subdomains, our results for IL-2-dependent T cells demonstrate that the membrane-proximal 52 amino acids are sufficient to mediate a normal proliferative response, including induction of the proto-oncogenes c-myc and c-fos. Although gamma c is phosphorylated on tyrosine upon receptor activation and could potentially interact with downstream molecules containing SH2 domains, cytoplasmic tyrosine residues were dispensable for mitogenic signalling. However, deletion of a membrane-proximal region conserved among other cytokine receptors (cytoplasmic residues 5 to 37) or an adjacent region unique to gamma c (residues 40 to 52) abrogated functional interaction of the receptor chain with the tyrosine kinase Jak3. This correlated with a loss of all signalling events analyzed, including phosphorylation of the IL-2R beta-associated kinase Jak1, expression of c-myc and c-fos, and induction of the proliferative response. Thus, it appears in T cells that Jak3 is a critical mediator of mitogenic signaling by the gamma c chain.

Roles of Protein-tyrosine Phosphatases in Stat1α-mediated Cell Signaling

Different Stat proteins are activated through phosphorylation of unique tyrosine residues in response to different cytokines and growth factors. Interferon-gamma activates Stat1 molecules that form homodimers and bind cognate DNA elements. Here we show that treatment of permeabilized cells with 200-500 microM peroxo-derivatives of vanadium, molybdenum, and tungsten results in the accumulation of constitutively phosphorylated Stat1 alpha molecules. In contrast, treatment of permeabilized cells with orthovanadate, vanadyl sulfate, molybdate, and tungstate at the same range of concentrations does not result in the accumulation of activated Stat1 alpha molecules in the absence of ligand. However, these compounds inhibit the inactivation of interferon-gamma-induced DNA-binding activity of Stat1 alpha. A 4-6-h exposure of the permeabilized cells to orthovanadate, molybdate, and tungstate, but not vanadyl sulfate, results in a ligand-independent activation of Stat1 alpha, which is blocked by the inhibition or depletion of NADPH oxidase activity in the cells, indicating that NADPH oxidase-catalyzed superoxide formation is required for the bioconversion of these metal oxides to the corresponding peroxo-compounds. Interestingly, ligand-independent Stat1 alpha activation by peroxo-derivatives of these transition metals does not require Jak1, Jak2, or Tyk2 kinase activity, suggesting that other kinases can phosphorylate Stat1 alpha on tyrosine 701.

JAK3 Protein Tyrosine Kinase Mediates Interleukin-7-Induced Activation of Phosphatidylinositol-3' Kinase

The interleukin-7 (IL-7) receptor is expressed throughout T-cell differentiation and, although lacking a tyrosine kinase domain, mediates tyrosine phosphorylation in T cells. We have identified IL-7-induced activation of three cyoplasmic tyrosine kinases in T cells, Jak1, Jak3, and the src-like kinase p56lck. Many members of the cytokine receptor superfamily activate the Jak protein tyrosine kinase family, with resultant phosphorylation of the Stat transcriptional activator factors. We describe here a novel function of the Jak kinases, because Jak kinase activity is not only required for Stat activation but also for P13 kinase response to IL-7 in human T cells. We show that IL-7 receptor-mediated Jak activation can occur independently of p56lck activity. IL-7-induced P13 kinase activation, mediated by tyrosine phosphorylation of the P13 kinase p85 subunit, is essential to the IL-7 proliferative signal and also occurs in the absence of src family kinase activity. Jak3 is found associated with the p85 subunit of P13 kinase in an IL-7-responsive manner in T cells and appears to regulate IL-7-induced P13 kinase activation by mediating tyrosine phosphorylation of the p85 subunit. Specific inhibition of IL-7-induced Jak kinase activity ablates p85 tyrosine phosphorylation, subsequent P13 kinase activation, and, ultimately, proliferation. The ability to regulate P13 kinase activity indicates a more generalized role for the Jak family than activation of gene transcription via the Stat family in cytokine receptor signal transduction.

Interleukin-6 and ciliary neurotrophic factor trigger Janus kinase activation and early gene response in rat hepatocytes

Interleukin-6 (IL-6) and ciliary neurotrophic factor (CNTF) share a signal-transducing molecule called gp130. Previous studies showed that CNTF regulates fibrinogen gene expression in rat hepatocytes by competitive binding to the IL-6 receptor. This report explores the post ligand-binding events in the control of fibrinogen and early response gene production stimulated by IL-6 and CNTF in primary rat hepatocytes. Metabolic labeling, using [ 32P]orthophosphate or anti-phosphotyrosine antibody (Ab) blot experiments revealed that both IL-6 and CNTF induced tyrosine phosphorylation of gp130, and the Jakl and Jak2 kinases in a dose- and time-dependent manner. Additional experiments revealed that only one of the early response genes, junb, but not c-myc or c-fos, was stimulated by the addition of either IL-6 or CNTF. These data suggest that activation of Jak kinases and stimulation of junb reflect a divergence of the IL-6/CNTF signaling pathway and further suggest that junb may participate in cytokine control of acute-phase protein production in the inflammatory response.

The Jak Kinases Differentially Associate with the α and β (Accessory Factor) Chains of the Interferon γ Receptor to Form a Functional Receptor Unit Capable of Activating STAT Transcription Factors

Interferon gamma (IFN gamma) induces the expression of early response genes by tyrosine phosphorylation of Jak kinases and transcription factors referred to as STAT proteins. The topology of the IFN gamma receptor is partially understood and the relationship between the alpha chain that binds the ligand and the beta chain that is required for signal transduction is undefined. In a cell line which expresses only the human alpha chain, we show that these cells did not activate Jak kinases or STAT proteins with human IFN gamma, even though Jak1 co-immunoprecipitated with the alpha chain. In cells unexposed to IFN gamma, Jak1 preferentially associated with the alpha chain, while Jak2 associated with the beta chain. There was evidence for Jak1 kinase activity in untreated cells. For Jak2, kinase activity was IFN gamma-dependent. Although the alpha chain was tyrosine-phosphorylated in response to ligand, we found no evidence for tyrosine phosphorylation of the beta chain. These data are consistent with a model of the IFN gamma receptor in which Jak1 associates with the alpha chain, whereas Jak2 associates with the beta chain. IFN gamma clusters at least two receptor units which results in the tyrosine phosphorylation of Jak1 and Jak2, the activation of Jak2 kinase activity, and the recruitment of STAT1 alpha resulting in its activation by tyrosine phosphorylation.

Activation of Stat5 by interleukin 2 requires a carboxyl-terminal region of the interleukin 2 receptor beta chain but is not essential for the proliferative signal transmission.

The high-affinity interleukin 2 (IL-2) receptor (IL-2R) consists of three subunits: the IL-2R alpha, IL-2R beta c, and IL-2R gamma c chains. Two members of the Janus kinase family, Jak1 and Jak3, are associated with IL-2R beta c and IL-2R gamma c, respectively, and they are activated upon IL-2 stimulation. The cytokine-mediated Jak kinase activation usually results in the activation of a family of latent transcription factors termed Stat (signal transducer and activator of transcription) proteins. Recently, the IL-2-induced Stat protein was purified from human lymphocytes and found to be the homologue of sheep Stat5/mammary gland factor. We demonstrate that the human Stat5 is activated by IL-2 and that Jak3 is required for the efficient activation. The cytoplasmic region of the IL-2R beta c chain required for activation of Stat5 is mapped within the carboxyl-terminal 147 amino acids. On the other hand, this region is not essential for IL-2-induced cell proliferation.

Tyrosine Phosphorylation and Activation of JAK Family Tyrosine Kinases by Interleukin-9 in MO7E Cells

Interleukin-9 (IL-9) is a T-cell-derived multifunctional cytokine that can stimulate the proliferation of a human megakaryocytic leukemia cell line, MO7E. Previous studies suggested that protein tyrosine phosphorylation may be involved in IL-9 signaling pathways. However, tyrosine kinases activated by IL-9 have not been identified. In this report we show that IL-9 induces tyrosine phosphorylation and activation of the JAK family tyrosine kinases including JAK1, JAK3, and Tyk2. The kinetic studies indicate that tyrosine phosphorylation and activation of JAK kinases induced by IL-9 occurred within 1 minute, peaked by 5 to 10 minutes, and persisted at least for 45 minutes. Furthermore, we show that signal transducers and activators of transcription (Stat) 91 or related protein and an 88-kD Stat 91-associated protein are rapidly tyrosine phosphorylated following IL-9 treatment. Gel shift assays confirm that nuclear extracts from MO7E cells stimulated with IL-9 specifically interact with a DNA element termed gamma activated site. These results suggest that actions of IL-9 may, in part, be mediated through JAK kinase-Stat signaling cascades.

Growth hormone receptor C-terminal domains required for growth hormone-induced intracellular free Ca2+ oscillations and gene transcription.

The biological effects of growth hormone (GH) are initiated by its binding to the GH receptor (GHR) followed by association and activation of the tyrosine kinase JAK2. Here we report that GH can stimulate an increase in intracellular free Ca2+ concentration ([Ca2+]i) in cells expressing wild-type GHRs and receptor mutants lacking up to 132 amino acids of the C terminus, whereas GHRs lacking a further 52 amino acids in the C terminus are unable to induce Ca2+ signaling. The GH-induced rise in [Ca2+]i was dependent upon extracellular Ca2+ and the response consisted of GH-induced Ca2+ oscillations of varying frequency and amplitude. GH-induced transcription of the serine protease inhibitor 2.1 gene required the same C-terminal 52-amino acid domain of the receptor as for Ca2+ signaling. Mutation of the four proline residues in the conserved box 1 region of the GHR, which is responsible for binding and activation of JAK2 kinase, completely abolished GH-induced gene transcription but did not affect the GH-induced rise in [Ca2+]i. The Ca2+ channel blocker verapamil prevented GH-induced Ca2+ signaling as well as GH-induced gene transcription in cells expressing endogenous GHRs. These findings indicate that the GHR can initiate two independent signaling pathways, one requiring the box 1 region and the other requiring the region between amino acids 454 and 506, and suggest that both of these pathways are required for GH-induced gene transcription.

Interleukin-5 Signaling in Human Eosinophils Involves JAK2 Tyrosine Kinase and Stat1α

Signaling by a wide variety of cytokines, including interferons, interleukins, and growth factors, involves activation of JAK kinases and Stat (Signal transducers and activators of transcription) proteins. At present, not much is known about the molecular mechanisms by which interleukin-5 (IL-5) exerts its diverse biologic effects. Human eosinophils are one of the most important target cells for IL-5 and were used here to study IL-5 signaling in a primary human cell. IL-5 induced rapid and transient tyrosine phosphorylation of JAK2. Moreover, IL-5 induced at least two DNA-binding complexes, using nuclear extracts from normal human eosinophils and the IL-6/interferon-gamma response element of the ICAM-1 promoter (ICAM-1 pIRE) in an electromobility shift assay. From supershift experiments it was concluded that one DNA-binding complex contained Stat1 alpha, probably as a homodimer. Both DNA-binding complexes were inhibited by a phosphotyrosine antibody (4G10), suggesting that tyrosine phosphorylation is required for complex formation. IL-3 and granulocyte-macrophage colony-stimulating factor induced, similar to IL-5, two DNA-binding complexes in human eosinophils, including Stat1 alpha. These data show for the first time that molecular mechanisms of IL-5 signaling in human eosinophils involve members of the JAK kinase family as well as members of the Stat family.

Phosphorylation and internalization of gp130 occur after IL-6 activation of Jak2 kinase in hepatocytes.

Recent evidence has shown that members of the Jak kinase family are activated after IL-6 binds to its receptor complex, leading to a tyrosine phosphorylation of gp130, the IL-6 signal-transducing subunit. The different members of the IL-6 cytokine subfamily induce distinct patterns of Jak-Tyk phosphorylation in different cell types. Using monospecific antibodies to gp130, Jak2 kinase, and phosphotyrosine, we investigated the kinetics of IL-6 stimulation of members of this pathway in primary hepatocytes. Our findings show that Jak 2 is maximally activated within 2 min of exposure to IL-6, followed by gp130 phosphorylation that reaches its peak in another 2 min then declines to basal level by 60 min. In vitro phosphorylation experiments show that activated Jak 2 is able to phosphorylate both native gp130 and a fusion peptide containing its cytoplasmic domain, demonstrating gp130 is a direct substrate of Jak 2 kinase. Experiments designed to explore the cell surface expression of gp130 show that > or = 2 h are required to get a second round of phosphorylation after the addition of more cytokines. This finding suggests that activated gp130 is internalized from the cell surface after IL-6 stimulation. Additional experiments using protein synthesis inhibitors reveal that new protein synthesis is required to get a second cycle of gp130 phosphorylation indicating gp130 must be synthesized de novo and inserted into the membrane. These findings provide strong evidence that down regulation of the IL-6 signal in hepatocytes involves the internalization and cytosol degradation of gp130.

Activation of JAK2 kinase mediated by the interleukin 6 signal transducer gp130.

The interleukin 6 receptor-associated signal transducer, gp130, is shared by receptor complexes for leukemia inhibitory factor, oncostatin M, ciliary neurotrophic factor, and interleukin 11. We show here that JAK2 kinase is rapidly tyrosine phosphorylated in mouse embryonic stem cells whose pluripotentiality is maintained only by gp130-sharing cytokines after stimulation that is known to induce gp130 homodimerization. JAK1 is also tyrosine phosphorylated, but to a lesser extent, under the same conditions. Comparable results are obtained with hemopoietic lineage cells such as myeloid leukemic M1 cells and pro-B-cell line-derived transfectants expressing gp130, the former of which differentiate into macrophages after stimulation of gp130 and the latter of which initiate DNA synthesis. gp130-dimerizing stimulus upregulates kinase activity of JAK2 as revealed by immunocomplex kinase assay. Deletion or point mutation in the membrane-proximal cytoplasmic motifs in gp130 that are conserved in the hemopoietic cytokine receptor family results in the loss of tyrosine phosphorylation of JAK2, which coincides with the lack of signal transducing capability of gp130 mutants.