hopscotch

EVOLUTIONARY HOMOLOGS

The catalytic domain of Hopscotch shows high homology to Elk, Jak1 and Fes receptor tyrosine kinases with identity on the order of 50%. Jak 1 is the prototype of Jak (Janus kinase) subfamily of non-receptor tyrosine kinases. Fes is the transforming protein of a strain of feline sarcoma virus (Binari, 1994).

Cytokines are critically important for the growth and development of a variety of cells. Janus kinases (JAKs) associate with cytokine receptors and are essential for transmitting downstream cytokine signals. However, the regulation of the enzymatic activity of the JAKs is not well understood. The role of tyrosine phosphorylation of JAK3 in regulating its kinase activity was investigated by analyzing mutations of tyrosine residues within the putative activation loop of the kinase domain. Specifically, tyrosine residues 980 and 981 of JAK3 were mutated to phenylalanine, either individually or doubly. JAK3 is autophosphorylated on multiple sites including Y980 and Y981. Compared with the activity of wild-type (WT) JAK3, mutant Y980F demonstrates markedly decreased kinase activity; optimal phosphorylation of JAK3 on other sites is dependent on Y980 phosphorylation. The mutant Y980F also exhibits reduced phosphorylation of its substrates, gammac and STAT5A. In contrast, mutant Y981F has greatly increased kinase activity, whereas the double mutant, YY980/981FF, show intermediate activity. These results indicate that Y980 positively regulates JAK3 kinase activity whereas Y981 negatively regulates JAK3 kinase activity. These observations in JAK3 are similar to the findings for the kinase that is closely related to the JAK family: ZAP-70. Mutations of tyrosine residues within the putative activation loop of ZAP-70 also have opposing actions. Thus, it will be important to determine whether this feature of regulation is unique to JAK3 or if it is also a feature of other JAKs. Given the importance of JAKs, and particularly JAK3, it will be critical to fully dissect the positive and negative regulatory function of these and other tyrosine residues in the control of kinase activity and hence cytokine signaling (Zhou, 1997).

Janus kinase 2 (JAK2), and more specifically just its intact N-terminal domain, binds to the erythropoietin receptor (EpoR) in the endoplasmic reticulum and promotes its cell surface expression. This interaction is specific as JAK1 has no effect. Residues 32 to 58 of the JAK2 JH7 domain are required for EpoR surface expression. Alanine scanning mutagenesis of the EpoR membrane proximal region reveals two modes of EpoR-JAK2 interaction. A continuous block of EpoR residues is required for functional, ligand-independent binding to JAK2 and cell surface receptor expression, whereas four specific residues are essential in switching on prebound JAK2 after ligand binding. Thus, in addition to its kinase activity required for cytokine receptor signaling, JAK is also an essential subunit required for surface expression of cytokine receptors (Huang, 2001).

In vascular smooth muscle cells, the induction of early growth response genes involves the Janus kinase (JAK)/signal transducer and activators of transcription (STAT) and the Ras/Raf-1/mitogen-activated protein kinase cascades. Electroporation of antibodies against MEK1 or ERK1 abolishes vascular smooth muscle cell proliferation in response to either platelet-derived growth factor or angiotensin II. However, anti-STAT1 or -STAT3 antibody electroporation abolishes proliferative responses only to angiotensin II and not to platelet-derived growth factor. AG-490, a specific inhibitor of the JAK2 tyrosine kinase, prevents proliferation of vascular smooth muscle cells, complex formation between JAK2 and Raf-1, the tyrosine phosphorylation of Raf-1, and the activation of ERK1 in response to either angiotensin II or platelet-derived growth factor. However, AG-490 has no effect on angiotensin II- or platelet-derived growth factor-induced Ras/Raf-1 complex formation. These results indicate that (1) STAT proteins play an essential role in angiotensin II-induced vascular smooth muscle cell proliferation, (2) JAK2 plays an essential role in the tyrosine phosphorylation of Raf-1, and (3) convergent mitogenic signaling cascades involving the cytosolic kinases JAK2, MEK1, and ERK1 mediate vascular smooth muscle cell proliferation in response to both growth factor and G protein-coupled receptors (Marrero, 1997).

Cytokines mediate a variety of effector cell functions, including cellular proliferation, differentiation, and modulation of the immune response. Many cytokines activate receptor-associated Janus kinases (JAKs) that promote tyrosine phosphorylation of signal transducers and activators of transcription (STAT) factors. Although JAK activation has been correlated with phosphorylation, the role of this tyrosine phosphorylation in the regulation of JAK1 and JAK3 remains unclear. The relative roles of JAK1 and JAK3 in the activation of STAT5 by interleukin-2 (IL-2) also remain poorly understood. Two conserved tyrosine residues were targeted within the activation loop of the JAK1 and JAK3 kinase domains for substitution with phenylalanines. In an overexpression system, the catalytic function of JAK1 strictly requires the presence of the first of these tyrosines, Y1033. In contrast, JAK3 retains catalytic activity when either or both of these activation-loop tyrosines are mutated. Analysis of JAK1/3 chimeras demonstrate that JAK activity is also controlled by intramolecular interactions involving the amino-terminal domain of the JAK as well as by the inherent signaling properties of the kinase domain. IL-2-dependent STAT5 induction was reconstiuted in a cell line that lacks detectable expression of JAK1 and JAK3. Catalytically active versions of both JAK1 and JAK3 must be present for effective induction of STAT5. It is concluded that JAK1 and JAK3 are differentially regulated by specific tyrosines within their respective activation loops. Additionally, the amino-terminal domain of JAK3 appears to contain regulatory sequences that modify the function of the kinase domain. Finally, both JAK1 and JAK3 must retain catalytic function for IL-2-induced STAT5 activation (Liu, 1997).

Exposure of cells to protein tyrosine phosphatase (PTP) inhibitors causes an increase in the phosphotyrosine content of many cellular proteins. However, the level at which the primary signaling event is affected is still unclear. Jaks are activated by tyrosine phosphorylation in cells that are briefly exposed to the PTP inhibitor pervanadate (PV), resulting in tyrosine phosphorylation and functional activation of Stat6 (in addition to other Stats). Mutant cell lines that lack Jak1 activity fail to support PV-mediated (or interleukin 4 [IL-4]-dependent) activation of Stat6 but can be rescued by complementation with functional Jak1. The docking sites for both Jak1 and Stat6 reside in the cytoplasmic domain of the IL-4 receptor alpha-chain (IL-4Ralpha). The glioblastoma-derived cell lines T98G, GRE, and M007, which do not express the IL-4Ralpha chain, fail to support Stat6 activation in response to either IL-4 or PV. Complementation of T98G cells with the IL-4Ralpha restores both PV-mediated and IL-4-dependent Stat6 activation. Murine L929 cells, which do not express the gamma common chain of the IL-4 receptor, support PV-mediated (but not IL-4-dependent) Stat6 activation. Thus, Stat6 activation by PV is an IL-4Ralpha-mediated, Jak1-dependent event that is independent of receptor dimerization. It is proposed that receptor-associated constitutive PTP activity functions to down-regulate persistent, receptor-linked kinase activity. Inhibition or deletion of PTP activity results in constitutive activation of cytokine signaling pathways (Haque, 1997).

Coexpression of the alpha and betaL subunits of the human interferon alpha (IFNalpha) receptor is required for the induction of an antiviral state by human IFNalpha. To explore the role of the different domains of the betaL subunit in IFNalpha signaling, the wild-type alpha subunit and truncated forms of the betaL chain were coexpressed in L-929 cells. The first 82 amino acids (AAs) (AAs 265-346) of the cytoplasmic domain of the betaL chain are sufficient to activate the Jak-Stat pathway and trigger an antiviral state after IFNalpha2 binding to the receptor. This region of the betaL chain, required for Jak1 binding and activation, contains the Box 1 motif that is important for the interaction of some cytokine receptors with Jak kinases. However, using glutathione S-transferase fusion proteins containing amino- and carboxyl-terminal deletions of the betaL cytoplasmic domain, it was demonstrated that the main Jak1-binding region (corresponding to AAs 300-346 on the beta subunit) is distinct from the Box 1 domain (AAs 287-295) (Domanski, 1997).

Activation of the tyrosine kinase JAK2 is an essential step in cellular signaling by growth hormone (GH) as well as many other hormones and cytokines. Murine JAK2 has a total of 49 tyrosines; if phosphorylated, they could serve as docking sites for Src homology 2 (SH2) or phosphotyrosine binding domain-containing signaling molecules. Using a yeast two-hybrid screen of a rat adipocyte cDNA library, a splicing variant of the SH2 domain-containing protein SH2-B, designated SH2-Bbeta, was identified as a JAK2-interacting protein. The carboxyl terminus of SH2-Bbeta (SH2-Bbetac), which contains the SH2 domain, specifically interacts with kinase-active, tyrosyl-phosphorylated JAK2 but not kinase-inactive, unphosphorylated JAK2 in the yeast two-hybrid system. In COS cells coexpressing SH2-Bbeta or SH2-Bbetac and murine JAK2, both SH2-Bbetac and SH2-Bbeta coimmunoprecipitate to a significantly greater extent with wild-type, tyrosyl-phosphorylated JAK2 than with kinase-inactive, unphosphorylated JAK2. In a far Western blot, SH2-Bbetac also binds to immunoprecipitated wild-type but not kinase-inactive JAK2 . In 3T3-F442A cells, growth hormone (GH) stimulates the interaction of SH2-Bbeta with tyrosyl-phosphorylated JAK2 both in vitro, as assessed by binding of JAK2 in cell lysates to glutathione S-transferase (GST)-SH2-Bbetac or GST-SH2-Bbeta fusion proteins, and in vivo, as assessed by coimmunoprecipitation of JAK2 with SH2-Bbeta. GH promotes a transient and dose-dependent tyrosyl phosphorylation of SH2-Bbeta in 3T3-F442A cells, further suggesting the involvement of SH2-Bbeta in GH signaling. Consistent with SH2-Bbeta being a substrate of JAK2, SH2-Bbetac is tyrosyl phosphorylated when coexpressed with wild-type but not kinase-inactive JAK2 in both yeast and COS cells. SH2-Bbeta was also tyrosyl phosphorylated in response to gamma interferon, a cytokine that activates JAK2 and JAK1. These data suggest that GH-induced activation and phosphorylation of JAK2 recruits SH2-Bbeta and its associated signaling molecules into a GHR-JAK2 complex, thereby initiating some as yet unidentified signal transduction pathways. These pathways are likely to be shared by other cytokines that activate JAK2 (Rui, 1997).

GH exerts a variety of metabolic and growth-promoting effects. GH induces activation of the GH receptor (GHR)-associated cytoplasmic tyrosine kinase, JAK2, resulting in tyrosine phosphorylation of the GHR and activation of STAT (signal transducer and activator of transcription), Ras-mitogen-activated protein kinase, and phosphoinositol 3-kinase signaling pathways, among others. GH-stimulated tyrosine phosphorylation of insulin receptor substrate (IRS) proteins has been demonstrated in vitro and in vivo. IRS-1 (Drosophila homolog Chico) is a multiply phosphorylated cytoplasmic docking protein involved in metabolic and proliferative signaling by insulin, IL-4, and other cytokines, but the physiological role of IRS-1 in GH signaling is unknown. In murine 3T3-F442A pre-adipocytes, GH-dependent tyrosine phosphorylation of IRS-1 is observed as is specific GH-induced coimmunoprecipitation of IRS-1 with JAK2. This interaction was examined by in vitro affinity precipitation experiments with glutathione-S-transferase fusion proteins incorporating regions of rat IRS-1 and, as a source of JAK2, extracts of 3T3-F442A cells. Fusion proteins containing amino-terminal regions of IRS-1 that include the pleckstrin homology, phosphotyrosine-binding, and Shc and IRS-1 NPXY-binding domains, but not those containing other IRS-1 regions or glutathione-S-transferase alone, bind JAK2 from cell extracts. Tyrosine-phosphorylated JAK2 resulting from GH stimulation is included in the amino-terminal IRS-1 fusion precipitates; however, neither tyrosine phosphorylation of JAK2 nor treatment of cells with GH before extraction is necessary for the specific JAK2-IRS-1 interaction to be detected. In contrast, in this assay, specific insulin receptor association with the IRS-1 phosphotyrosine-binding, and Shc and IRS-1 NPXY-binding domains is insulin and phosphotyrosine dependent. To test for significance of IRS-1 with regard to GH signaling, IRS- and GHR-deficient 32D cells were stably reconstituted with the rabbit (r) GHR, either alone (32D-rGHR) or with IRS-1 (32D-rGHR-IRS-1). As assayed by three independent methods, GH induces proliferation in 32D-rGHR cells, even in the absence of transfected IRS-1. Notably, however, GH-induced proliferation is markedly enhanced in cells expressing IRS-1. Similarly, GH-induced mitogen-activated protein kinase activation is significantly augmented in IRS-1-expressing cells relative to that in cells harboring no IRS-1. These results indicate that IRS-1 enhances GH-induced proliferative signaling (Liang, 1999).

When growth hormone binds to its receptor, which belongs to the cytokine receptor superfamily, activates the Janus kinase Jak2. Jak2 provides tyrosine-kinase activity and initiates an activation of several key intracellular proteins (for example, mitogen-activated protein (MAP) kinases) that eventually execute the biological actions induced by growth hormone, including the expression of particular genes. In contrast to receptors that themselves have tyrosine kinase activity, the signaling pathways leading to MAP kinase activation triggered by growth hormone are poorly understood, but appear to be mediated by the proteins Grb2 and Shc. Growth hormone stimulates tyrosine phosphorylation of the receptor for epidermal growth factor (Egf-r) and its association with Grb2; at the same time, Egf-r stimulates MAP kinase activity in liver, an important target tissue of growth hormone. Expression of Egf-r and its mutants reveals that growth-hormone-induced activation of MAP kinase and expression of the transcription factor c-fos requires phosphorylation of tyrosines on Egf-r, but not its own intrinsic tyrosine-kinase activity. The tyrosine at residue 1,068 of the Egf-r is proposed to be one of the principal phosphorylation sites and a Grb2-binding site stimulated by growth hormone via Jak2. These results indicate that the role of Egf-r in signaling by growth hormone is to be phosphorylated by Jak2, thereby providing docking sites for Grb2 and activating MAP kinases and gene expression, independent of the intrinsic tyrosine kinase activity of Egf-r. This may represent a novel cross-talk pathway between the cytokine receptor superfamily and growth factor receptor (Yamauchi, 1997).

The Janus family of protein tyrosine kinases (JAKs) regulates cellular processes involved in cell growth, differentiation and transformation through their association with cytokine receptors. However, compared with other kinases, little is known about cellular regulators of the JAKs. A JAK-binding protein (JAB) has been identified that inhibits JAK signaling in cells. JAB specifically binds to the tyrosine residue (Y1007) in the activation loop of JAK2, whose phosphorylation is required for activation of kinase activity. Binding to the phosphorylated activation loop requires the JAB SH2 domain and an additional N-terminal 12 amino acids (extended SH2 subdomain) containing two residues (Ile68 and Leu75) that are conserved in JAB-related proteins. An additional N-terminal 12-amino-acid region (kinase inhibitory region) of JAB also contributes to high-affinity binding to the JAK2 tyrosine kinase domain and is required for inhibition of JAK2 signaling and kinase activity. These studies define a novel type of regulation of tyrosine kinases and might provide a basis for the design of specific tyrosine kinase inhibitors (Yasukawa, 1999).

A mechanism by which members of the ciliary neurotrophic factor (CNTF)-leukemia inhibitory factor cytokine family regulate gliogenesis in the developing mammalian central nervous system has been characterized. Activation of the CNTF receptor promotes differentiation of cerebral cortical precursor cells into astrocytes and inhibits differentiation of cortical precursors along a neuronal lineage. Although CNTF stimulates both the Janus kinase-signal transducer and activator of transcription (JAK-STAT) and Ras-mitogen-activated protein kinase signaling pathways in cortical precursor cells, the JAK-STAT signaling pathway selectively enhances differentiation of these precursors along a glial lineage. These findings suggest that cytokine activation of the JAK-STAT signaling pathway may be a mechanism by which cell fate is controlled during mammalian development (Bonni, 1997).

Jak2-deficient mice demonstrate an embryonic lethality due to the absence of definitive erythropoiesis. Fetal liver myeloid progenitors, although known to be present based on the expression of lineage specific markers, fail to respond to erythropoietin, thrombopoietin, interleukin-3 (IL-3), or granulocyte/macrophage colony-stimulating factor. In contrast, the response to granulocyte specific colony-stimulating factor is unaffected. Jak2-deficient fibroblasts fail to respond to interferon gamma (IFNgamma), although the responses to IFNalpha/beta and IL-6 are unaffected. Reconstitution experiments demonstrate that Jak2 is not required for the generation of lymphoid progenitors, their amplification, or functional differentiation. Therefore, Jak2 plays a critical, nonredundant role in the function of a specific group of cytokines receptors (Parganas, 1998).

Janus kinases (Jaks) play an important role in signal transduction via cytokine and growth factor receptors. To date, it is known that Jak2 is associated with receptors binding to the following factors: erythropoietin, growth hormone and thrombopoietin, as well as common beta chain-containing receptors known to bind granulocyte/macrophage colony-stimulating factor (GM-CSF), interleukin-3, and IL-5. Other receptors associated with Jak2 are cardiotrophin-receptor, prolactin-receptor, granulocyte CSF-R, and cytokine receptors containing gp130 in their receptor chain complex. A targeted inactivation of Jak2 was performed. Jak2-/- embryos are anemic and die around day 12.5 postcoitum. Primitive erythrocytes are found, but definitive erythropoiesis is absent. Compared to erythropoietin receptor-deficient mice, the phenotype of Jak2 deficiency is more severe. Fetal liver BFU-E and CFU-E colonies are completely absent. However, multilineage hematopoietic stem cells [CD34low, c-kit(pos)] can be found, and B lymphopoiesis appears intact. In contrast to IFNalpha stimulation, Jak2-/- cells do not respond to IFNgamma. Jak2-/- embryonic stem cells are competent for LIF signaling. The data provided demonstrate that Jak2 has pivotal functions for signal transduction of a set of cytokine receptors required in definitive erythropoiesis (Neubauer, 1998).

Mice lacking the ubiquitously expressed Janus kinase, Jak1, are runted at birth, fail to nurse, and die perinatally. Although Jak1-/- cells are responsive to many cytokines, they fail to manifest biologic responses to cytokines that bind to three distinct families of cytokine receptors. These include all class II cytokine receptors, cytokine receptors that utilize the gamma(c) subunit for signaling, and the family of cytokine receptors that depends on the gp130 subunit for signaling. These results thus demonstrate that Jak1 plays an essential and nonredundant role in promoting biologic responses induced by a select subset of cytokine receptors, including those in which Jak utilization was thought to be nonspecific (Rodig, 1998).

Leptin exerts its weight-reducing effects by binding to its receptor and activating signal transduction in hypothalamic neurons and other cell types. To identify the components of the leptin signal transduction pathway, an approach was developed in which bacterially expressed phosphorylated fragments of Ob receptor b (Ob-Rb) were used as affinity agents. Leptin binding to the Ob-Rb form of the leptin receptor leads to tyrosyl phosphorylation of the cytoplasmic domain of its receptor. Two of the three cytoplasmic tyrosines of Ob-Rb, at positions 985 and 1138, are phosphorylated after leptin treatment. Affinity chromatography using a tyrosine-phosphorylated fragment spanning Tyr 985 of Ob-Rb was used to identify proteins that bind to this site. The SH2 domain containing protein tyrosine phosphatase 2 (SHP-2) was isolated from bovine and mouse hypothalamus by using this method. After cotransfection into 293T cells of Ob-Rb, Janus kinase 2 (JAK2), and SHP-2, leptin treatment results in direct binding of SHP-2 to the phosphorylated Tyr 985. The bound SHP-2 is itself tyrosine phosphorylated after leptin treatment. SHP-2 is not phosphorylated after leptin treatment when a Y to F 985 receptor mutant is cotransfected. In the absence of SHP-2 phosphorylation, the level of JAK2 phosphorylation is increased. Tyrosyl phosphorylation of the leptin receptor and signal transducer and activator of transcription 3 (STAT3) are not affected by phosphorylation of SHP-2. These data suggest that activation of SHP-2 by the leptin receptor results in a decreased phosphorylation of JAK2 and may act to attenuate leptin signal transduction. The data also suggest that the dephosphorylation of JAK2 is a direct action of SHP-2. Thus, a point mutation that ablates SHP-2 phosphatase activity also ablates its effects on the state of JAK2 phosphorylation. Although SHP-2 does have intrinsic phosphatase activity, it also could lead to dephosphorylation of JAK2 indirectly by functioning as an adapter protein. For example, binding of SHP-2 to the activated platelet-derived growth factor receptor leads to its own phosphorylation at position Tyr 584, which in turn leads to binding of Grb2. Grb2 then activates ras and the mitogen-activated protein kinase signaling pathway. Previous studies have shown that leptin can activate mitogen-activated protein kinase. Indeed the available data are consistent with the possibility that SHP-2 could both decrease JAK2 phosphorylation and stimulate signaling via the mitogen-activated protein kinase or other pathways. The method used in this report can in principle be used to isolate additional components of the leptin, or other, signal transduction pathway (Li, 1999).

What is the effect of the v-abl oncogene (Drosophila homolog: see enabled) of the Abelson murine leukemia virus (A-MuLV) on the Jak-STAT pathway of cytokine signal transduction? In murine pre-B lymphocytes transformed with A-MuLV, the Janus kinases (Jaks) Jak1 and Jak3 exhibit constitutive tyrosine kinase activity, and the STAT proteins (signal transducers and activators of transcription) normally activated by interleukin-4 and interleukin-7 are tyrosine-phosphorylated in the absence of these cytokines. Coimmunoprecipitation experiments reveal that in these cells v-Abl is physically associated with Jak1 and Jak3. Inactivation of v-Abl tyrosine kinase in a pre-B cell line transformed with a temperature-sensitive mutant of v-abl results in abrogation of constitutive Jak-STAT signaling. A direct link may exist between transformation by v-abl and cytokine signal transduction (Danial, 1995).

In Abelson murine leukemia virus (A-MuLV)-transformed cells, members of the Janus kinase (Jak) family of non-receptor tyrosine kinases and the signal transducers and activators of transcription (STAT) family of signaling proteins are constitutively activated. In these cells, the v-Abl oncoprotein and the Jak proteins physically associate. To define the molecular mechanism of constitutive Jak-STAT signaling in these cells, the functional significance of the v-Abl-Jak association was examined. Mapping the Jak1 interaction domain in v-Abl demonstrates that amino acids 858 to 1080 within the carboxyl-terminal region of v-Abl bind Jak1 through a direct interaction. A mutant of v-Abl lacking this region exhibits a significant defect in Jak1 binding in vivo, fails to activate Jak1 and STAT proteins, and does not support either the proliferation or the survival of BAF/3 cells in the absence of cytokine. Cells expressing this v-Abl mutant show extended latency and decreased frequency in generating tumors in nude mice. In addition, inducible expression of a kinase-inactive mutant of Jak1 protein inhibits the ability of v-Abl to activate STATs and to induce cytokine-independent proliferation, indicating that an active Jak1 is required for these v-Abl-induced signaling pathways in vivo. It is proposed that Jak1 is a mediator of v-Abl-induced STAT activation and v-Abl induced proliferation in BAF/3 cells, and may be important for efficient transformation of immature B cells by the v-abl oncogene (Danial, 1998).

Human T cell leukemia/lymphotropic virus type I (HTLV-I) induces adult T cell leukemia/lymphoma (ATLL). The mechanism of HTLV-I oncogenesis in T cells remains partly elusive. In vitro, HTLV-I induces ligand-independent transformation of human CD4(+) T cells, an event that correlates with acquisition of constitutive phosphorylation of Janus kinases (JAK) and signal transducers and activators of transcription (STAT) proteins. However, it is unclear whether the in vitro model of HTLV-I transformation has relevance to viral leukemogenesis in vivo. In cell extracts of uncultured leukemic cells from 12 patients with ATLL, the status of both JAK/STAT phosphorylation and DNA-binding activity of STAT proteins was tested with DNA-binding assays, using DNA oligonucleotides specific for STAT-1 and STAT-3, STAT-5 and STAT-6 or, more directly, by immunoprecipitation and immunoblotting with anti-phosphotyrosine antibody for JAK and STAT proteins. Leukemic cells from 8 of the 12 patients studied displayed constitutive DNA-binding activity of one or more STAT proteins; the constitutive activation of the JAK/STAT pathway was found to persist over time in the 2 patients followed longitudinally. An association between JAK3 and STAT-1, STAT-3, and STAT-5 activation and cell-cycle progression has been demonstrated by both propidium iodide staining and bromodeoxyuridine incorporation in cells of four of the patients tested. These results imply that JAK/STAT activation is associated with replication of leukemic cells and that therapeutic approaches aimed at JAK/STAT inhibition may be considered to halt neoplastic growth (Takemoto, 1997).

Stromal cell-derived factor-1 (SDF-1), the ligand for the CXCR4 receptor, is a highly efficacious chemoattractant for CD34(+) hematopoietic progenitor cells. However, the SDF-1/CXCR4 signaling pathways that regulate hematopoiesis are still not well defined. This study reports that SDF-1alpha can stimulate the tyrosine phosphorylation of Janus kinase 2 (JAK2) and other members of the JAK/signal transduction and activation of transcription (STAT) family, including JAK1, tyrosine kinase 2, STAT2, and STAT4 in the human progenitor cell line, CTS. SDF-1alpha stimulation of these cells also enhances the association of JAK2 with phosphatidylinositol 3 (PI3)-kinase. This enhanced association is abolished by pretreatment of cells with AG490, a specific JAK2 inhibitor. Furthermore, pretreatment of CTS cells with AG490 significantly inhibits SDF-1alpha-induced PI3-kinase activity, and inhibition of JAK2 with AG490 ablates the SDF-1alpha-induced tyrosine phosphorylation of multiple focal adhesion proteins (including focal adhesion kinase, related adhesion focal tyrosine kinase, paxillin, CrkII, CrkL, and p130Cas; see CAS/CSE1 segregation protein). Chemotaxis assays show that inhibition of JAK2 diminishes SDF-1alpha-induced migration in both CTS cells and CD34(+) human bone marrow progenitor cells. Hence, these results suggest that JAK2 is required for CXCR4 receptor-mediated signaling that regulates cytoskeletal proteins and cell migration through PI3-kinase pathways in hematopoietic progenitor cells (Zhang, 2001).

Defining signals that can support the self-renewal of multipotential hemopoietic progenitor cells (MHPCs) is pertinent to understanding leukemogenesis and may be relevant to developing stem cell-based therapies. A set of signals, JAK2 plus either c-kit or flt-3, is defined which together can support extensive MHPC self-renewal. Phenotypically and functionally distinct populations of MHPCs are obtained, depending on which receptor tyrosine kinase, c-kit or flt-3, is activated. Self-renewal is abrogated in the absence of STAT5a/b, and in the presence of inhibitors targeting either the mitogen-activated protein kinase or phosphatidylinositol 3' kinase pathways. These findings suggest that a simple two-component signal can drive MHPC self-renewal (Zhao, 2002).

JAK-STAT signalling is required throughout telotrophic oogenesis and short-germ embryogenesis of the beetle Tribolium

In Drosophila, the JAK-STAT signalling pathway regulates a broad array of developmental functions including segmentation and oogenesis. This study analysed the functions of Tribolium JAK-STAT signalling factors and of Suppressor Of Cytokine Signalling (SOCS) orthologues, which are known to function as negative regulators of JAK-STAT signalling, during telotrophic oogenesis and short-germ embryogenesis. The beetle Tribolium features telotrophic ovaries, which differ fundamentally from the polytrophic ovary of Drosophila. While the requirement for JAK-STAT signalling in specifying the interfollicular stalk was found to be principally conserved, it was demonstrated that these genes also have early and presumably telotrophic specific functions. Moreover, the SOCS genes crucially contribute to telotrophic Tribolium oogenesis, as their inactivation by RNAi results in compound follicles. During short-germ embryogenesis, JAK-STAT signalling is required in the maintenance of segment primordia, indicating that this signalling cascade acts in the framework of the segment-polarity network. In addition, it is demonstrated that JAK-STAT signalling crucially contributes to early anterior patterning. It is posited that this signalling cascade is involved in achieving accurate levels of expression of individual pair-rule and gap gene domains in early embryonic patterning (Bäumer, 2011).

JAK signaling globally counteracts heterochromatic gene silencing

The JAK/STAT pathway has pleiotropic roles in animal development, and its aberrant activation is implicated in multiple human cancers. JAK/STAT signaling effects have been attributed largely to direct transcriptional regulation by STAT of specific target genes that promote tumor cell proliferation or survival. In a Drosophila hematopoietic tumor model, however, that JAK overactivation globally disrupts heterochromatic gene silencing, an epigenetic tumor suppressive mechanism. This disruption allows derepression of genes that are not direct targets of STAT, as evidenced by suppression of heterochromatin-mediated position effect variegation. Moreover, mutations in the genes encoding heterochromatin components heterochromatin protein 1 (HP1) and Su(var)3-9 enhance tumorigenesis induced by an oncogenic JAK kinase without affecting JAK/STAT signaling. Consistently, JAK loss of function enhances heterochromatic gene silencing, whereas overexpressing HP1 suppresses oncogenic JAK-induced tumors. These results demonstrate that the JAK/STAT pathway regulates cellular epigenetic status and that globally disrupting heterochromatin-mediated tumor suppression is essential for tumorigenesis induced by JAK overactivation (Shi, 2006).

JAK2 phosphorylates histone H3Y41 and excludes HP1alpha from chromatin

Activation of Janus kinase 2 (JAK2) by chromosomal translocations or point mutations is a frequent event in haematological malignancies. JAK2 is a non-receptor tyrosine kinase that regulates several cellular processes by inducing cytoplasmic signalling cascades. This study shows that human JAK2 is present in the nucleus of haematopoietic cells and directly phosphorylates Tyr 41 (Y41) on histone H3. Heterochromatin protein 1alpha (HP1alpha), but not HP1beta, specifically binds to this region of H3 through its chromo-shadow domain. Phosphorylation of H3Y41 by JAK2 prevents this binding. Inhibition of JAK2 activity in human leukaemic cells decreases both the expression of the haematopoietic oncogene lmo2 and the phosphorylation of H3Y41 at its promoter, while simultaneously increasing the binding of HP1alpha at the same site. These results identify a previously unrecognized nuclear role for JAK2 in the phosphorylation of H3Y41 and reveal a direct mechanistic link between two genes, jak2 and lmo2, involved in normal haematopoiesis and leukaemia (Dawson, 2009).

JAK tyrosine kinases promote hierarchical activation of Rho and Rap modules of integrin activation

Lymphocyte recruitment is regulated by signaling modules based on the activity of Rho and Rap small guanosine triphosphatases that control integrin activation by chemokines. This study shows that Janus kinase (JAK) protein tyrosine kinases control chemokine-induced Lymphocyte function-associated antigen 1 (LFA-1) and Integrin alpha4beta1 (VLA-4) mediated adhesion as well as human T lymphocyte homing to secondary lymphoid organs. JAK2 and JAK3 isoforms, but not JAK1, mediate CXCL12-induced LFA-1 triggering to a high affinity state. Signal transduction analysis showed that chemokine-induced activation of the Rho module of LFA-1 affinity triggering is dependent on JAK activity, with VAV1 mediating Rho activation by JAKs in a Galphai-independent manner. Furthermore, activation of Rap1A by chemokines is also dependent on JAK2 and JAK3 activity. Importantly, activation of Rap1A by JAKs is mediated by RhoA and PLD1, thus establishing Rap1A as a downstream effector of the Rho module. Thus, JAK tyrosine kinases control integrin activation and dependent lymphocyte trafficking by bridging chemokine receptors to the concurrent and hierarchical activation of the Rho and Rap modules of integrin activation (Montresor, 2013).

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