Cytokine signaling through nonreceptor protein tyrosine kinases

A single tyrosine of the interleukin-9 (IL-9) receptor is required for STAT activation, antiapoptotic activity, and growth regulation by IL-9

Interleukin-9 (IL-9), a T-cell-derived cytokine, interacts with a specific receptor associated with the IL-2 receptor gamma chain. In this report, we analyze the functional domains of the human IL-9 receptor transfected into mouse lymphoid cell lines. Three different functions were examined: growth stimulation in factor-dependent pro-B Ba/F3 cells, protection against dexamethasone-induced apoptosis, and Ly-6A2 induction in BW5147 lymphoma cells. The results indicated that a single tyrosine, at position 116 in the cytoplasmic domain, was required for all three activities. In addition, we observed that human IL-9 reduced the proliferation rate of transfected BW5147 cells, an effect also dependent on the same tyrosine. This amino acid was necessary for IL-9-mediated tyrosine phosphorylation of the receptor and for STAT activation but not for IRS-2/4PS activation or for JAK1 phosphorylation, which depended on a domain closer to the plasma membrane. We also showed that JAK1 was constitutively associated with the IL-9 receptor. Activated STAT complexes induced by IL-9 were found to contain STAT1, STAT3, and STAT5 transcription factors. Moreover, sequence homologies between human IL-9 receptor tyrosine 116 and tyrosines (of other receptors activating STAT3 and STAT5 were observed. Taken together, these data indicate that a single tyrosine of the IL-9 receptor, required for activation of three different STAT proteins, is necessary for distinct activities of this cytokine, including proliferative responses.

Activation of Jak/STAT proteins involved in signal transduction pathway mediated by receptor for interleukin 2 in malignant T lymphocytes derived from cutaneous anaplastic large T-cell lymphoma and Sezary syndrome

Signaling through the interleukin 2 receptor (IL-2R) involves phosphorylation of several proteins including Jak3, STAT5, and, in preactivated cells, STAT3. In the present study, we examined the functional status of the IL-2R-associated Jak/STAT pathway in malignant T lymphocytes from advanced skin-based lymphomas: anaplastic large T-cell lymphoma (ALCL) and Sezary syndrome (SzS). Proliferation of three ALCL cell lines (PB-1, 2A, and 2B) was partially inhibited by rapamycin, a blocker of some of the signals mediated by IL-2R, but not by cyclosporin A, FK-506, and prednisone, which suppress signals mediated by the T-cell receptor. All the cell lines expressed on their surface the high-affinity IL-2R (alpha, beta, and gamma c chains). They showed basal, constitutive phosphorylation, and coassociation of Jak3, STAT5, and STAT3. Weak basal phosphorylation of IL-2R gamma c was also detected. In regard to SzS, peripheral blood mononuclear cells from 10 of 14 patients showed basal phosphorylation of Jak3, accompanied by phosphorylation of STAT5 in 9 patients, and STAT3 in 4 patients. However, in vitro overnight culture of SzS cells without exogenous cytokines resulted in markedly decreased Jak3 and STAT5 phosphorylation, which could be reversed by stimulation with IL-2. This indicates that the basal phosphorylation of Jak3 and STAT5 in freshly isolated SzS cells is induced rather than constitutive. The basal activation of the Jak/STAT pathway involved in IL-2R signal transduction in ALCL and SzS cells reported here suggests that this pathway may play a role in the pathogenesis of cutaneous T-cell lymphomas, although the mechanism (induced versus constitutive) may vary between different lymphoma types.

Contrasting acute in vivo nuclear actions of growth hormone and prolactin

Growth hormone (GH) and prolactin (PRL) exert long-term effects on cellular metabolism, growth, and development through changes in gene expression and protein biosynthesis that are initiated by hormone binding to specific cell-surface receptors. Recent studies have demonstrated that ligand-induced activation of both GH and PRL receptors leads to the tyrosine phosphorylation of multiple intracellular proteins by the identical non-receptor tyrosine kinase, JAK2. We have shown previously that in vivo administration of human recombinant GH rapidly stimulated the inducible transcription factors, Stats1, 3, and 5, and acutely altered gene transcription in the liver. Because human GH can bind to both lactogenic and somatogenic receptors with high affinity, in this study we have addressed the question of specificity of the hormonal response by examining the early nuclear events following a single injection of rat GH or rat PRL to hormone-deficient hypophysectomized female rats. We find that PRL stimulated tyrosine phosphorylation of Stat5, induced nuclear protein binding to the GH-responsive element of the serine protease inhibitor (Spi) 2.1 promoter, and activated Spi 2.1 gene expression. These acute actions of rat PRL were modest compared to the effects of rat GH. GH treatment induced tyrosine phosphorylation of several hepatic nuclear proteins, activated Stats1, 3, and 5, stimulated Spi 2.1 gene expression, and inhibited albumin gene transcription. All of the effects of rat GH paralleled responses to human GH that we have measured previously. Based on these results, it is likely that most of the actions of human GH in the liver are mediated by the GH receptor rather than by the PRL receptor. The diminished response to PRL may be secondary to the high density of short PRL receptor isoforms in the liver, which do not participate effectively in ligand-induced signal transmission.

Immunodeficiencies caused by genetic defects in protein kinases

The recognition that defects of ZAP-70 and, more recently, of JAK3 kinase in humans result in severe combined immunodeficiency, and the demonstration that targeting of these and other protein-kinase genes in mice also leads to immunodeficiency, have highlighted the crucial role that these proteins play in T-cell differentiation and activation.

The harmony of the spheres: inducible nitric oxide synthase and related genes in pancreatic beta cells

The radical nitric oxide (NO) is a possible mediator of pancreatic beta-cell damage in insulin-dependent diabetes mellitus (IDDM). NO is produced by the enzyme nitric oxide synthase (NOS), in a reaction where arginine is the main substrate. There are different isoforms of NOS, but in the context of immune mediated beta-cell damage the inducible form of NOS (iNOS) is the most relevant. The beta-cell iNOS is similar and encoded by the same gene on chromosome 17 as the iNOS expressed in macrophages and other nucleated cells. iNOS activation depends on gene transcription and de novo enzyme synthesis, and NO seems to induce a negative feedback on iNOS expression. While iNOS mRNA is induced by interleukin-1 beta (IL-1 beta) alone in rodent insulin-producing cells, a combination of two (IL-1 beta + interferon gamma) (IFN-gamma) or three (IL-1 beta + IFN gamma + tumour necrosis factor alpha) cytokines is required for iNOS activation in human pancreatic islets. The promoter region of the murine iNOS gene has at least 25 binding sites for different transcription factors, and the nuclear transcription factor kappa B is necessary for cytokine-induced iNOS transcription in both rodent and human pancreatic islets. The nature of other transcription factors relevant for iNOS regulation in these cells remains to be determined. Induction of iNOS is paralleled by induction of several other cytokine-dependent genes in beta cells, including argininosuccinate synthetase, cyclooxygenase and manganese superoxide dismutase. Some of these genes may contribute to beta-cell damage, while others are probably involved in beta-cell defence and/or repair. Regulation of iNOS and other related genes in beta cells is complex, and differs in several aspects from that observed in macrophages. There are also important differences in iNOS regulation between rodent and human pancreatic islets. A detailed knowledge of the molecular regulation of these genes in beta cells may be instrumental in the development of new approaches to prevent beta-cell destruction in early IDDM.

Multiple stat complexes interact at the interferon regulatory factor-1 interferon-gamma activation sequence in prolactin-stimulated Nb2 T cells

Interferon regulatory factor-1 (IRF-1) is a major immediate early gene induced by prolactin (PRL) in a biphasic, cell cycle-dependent manner in Nb2 T cells. This biphasic expression (30 min and 10 h) is mediated in part by an interferon-gamma activation sequence (GAS) in the IRF-1 promoter which binds factors belonging to the Signal Transducers and Activators of Transcription (Stat) family. By electrophoretic mobility shift assays (EMSA), Stat1 alpha was found to be the major and Stat5a a minor component of the 30 min complex. At 10 h, Stat-like factors were again found at the IRF-1 GAS. Western blot analyses show that Stat5a was rapidly induced by PRL to enter the nucleus, but unexpectedly, Stat1 alpha and the alternatively-spliced Stat1 beta were already present in the uninduced nucleus. Further, Stat1 alpha but not Stat1 beta is preferentially tyrosine phosphorylated in response to PRL stimulation. Our studies suggest that multiple Stat complexes may contribute to the biphasic transcription of the IRF-1 gene in PRL-stimulated T cells.

Other kinases can substitute for Jak2 in signal transduction by interferon-gamma

Each cytokine which utilizes the Jak-Stat signal transduction pathway activates a distinct combination of members of the Jak and Stat families. Thus, either the Jaks, the Stats, or both could contribute to the specificity of ligand action. With the use of chimeric receptors involving the interferon gamma receptor (IFN-gammaR) complex as a model system, we demonstrate that Jak2 activation is not an absolute requirement for IFN-gamma signaling. Other members of the Jak family can functionally substitute for Jak2. IFN-gamma can signal through the activation of Jak family members other than Jak2 as measured by Statlalpha homodimerization and major histocompatibility complex class I antigen expression. This indicates that Jaks are interchangeable and indiscriminative in the Jak-Stat signal transduction pathway. The necessity for the activation of one particular kinase during signaling can be overcome by recruiting another kinase to the receptor complex. The results may suggest that the Jaks do not contribute to the specificity of signal transduction in the Jak-Stat pathway to the same degree as Stats.

Autophosphorylation of the Fes tyrosine kinase. Evidence for an intermolecular mechanism involving two kinase domain tyrosine residues

The human c-fes proto-oncogene encodes a cytoplasmic tyrosine kinase (Fes) that is associated with multiple hematopoietic cytokine receptors. Fes tyrosine autophosphorylation sites may regulate kinase activity and recruit downstream signaling proteins with SH2 domains. To localize the Fes autophosphorylation sites, full-length Fes and deletion mutants lacking either the unique N-terminal or SH2 domain were autophosphorylated in vitro and analyzed by CNBr cleavage. Identical phosphopeptides of 10 and 4 kDa were produced with all three proteins, localizing the tyrosine autophosphorylation sites to the C-terminal kinase domain. Substitution of kinase domain tyrosine residues 713 or 811 with phenylalanine resulted in a loss of the 10- and 4-kDa phosphopeptides, respectively, identifying these tyrosines as in vitro autophosphorylation sites. CNBr cleavage analysis of Fes isolated from 32PO4-labeled 293T cells showed that Tyr-713 and Tyr-811 are also autophosphorylated in vivo. Mutagenesis of Tyr-713 reduced both autophosphorylation of Tyr-811 and transphosphorylation of Bcr, a recently identified Fes substrate, supporting a major regulatory role for Tyr-713. Wild-type Fes transphosphorylated a kinase-inactive Fes mutant on Tyr-713 and Tyr-811, suggesting that Fes autophosphorylation occurs via an intermolecular mechanism analogous to receptor tyrosine kinases.

The box1 domain of the erythropoietin receptor specifies Janus kinase 2 activation and functions mitogenically within an interleukin 2 beta-receptor chimera

Several distinct classes of cytokine receptors engage Jak kinases as primary effectors. Among type 1 receptors, Janus-activated kinase (Jak) recruitment is mediated by membrane-proximal cytoplasmic domains, which typically contain conserved box motifs. In the erythropoietin receptor (Epo-R), two such motifs (box1 and box2) have been suggested to be essential for the activation of Jak2 and mitogenesis. Presently, an Epo-R chimera containing the extracellular and box1 domains of the Epo-R (Jak2-associated receptor) and the box2 and carboxyl-terminal domains of the interleukin 2 beta-receptor (IL2beta-R; a Jak1-associated subunit) is shown to activate Jak2. Interestingly, Jak2 also was activated in FDC-P1 cells by a control Epo-R chimera containing the complete IL2beta-R cytoplasmic domain, and mitogenesis was supported by each of these above chimeras. By comparison, in BaF3 cells expressing IL2 receptor alpha and gamma subunits, an ectopically expressed IL2beta-R chimera containing the box1 domain of the Epo-R, activated Jak2 and Jak3 and was as mitogenically active as the wild-type IL2beta-R (Jak1 and Jak3 activation). Thus, the box1 domain of the Epo-R specifies Jak2 activation and functions efficiently within a heterologous IL2 receptor complex that normally activates Jak1 and Jak3.

Interleukin 7 receptor-deficient mice lack gammadelta T cells

The interleukin 7 receptor (IL-7R) plays a crucial role in early B- and T-cell development. It consists of a unique a chain and a common gamma chain [IL-2 receptor gamma chain (IL-2Rgamma)]. Gene inactivation of IL-7, IL-7R, and IL-2Rgamma resulted in severe impairment of B and T lymphopoiesis in mice. In addition, IL-2Rgamma-deficient mice lack gammadelta T cells in the skin and have the impaired development of natural killer (NK) cells and intraepithelial lymphocytes. To explore the role of IL-7/IL-7R system in gammadelta T- and NK-cell development, we have generated and analyzed IL-7R-deficient mice. gammadelta T cells were absent from skin, gut, liver, and spleen in the deficient mice. In contrast, alphabeta T and B cells were detected in reduced, but certain, numbers, and NK cells developed normally. The gammadelta T-cell development in fetal and adult thymus was also completely blocked. These results clearly demonstrate that the signal from IL-7R is indispensable for gammadelta T-cell development in both thymic and extrathymic pathways. On the contrary, it is suggested that NK-cell development requires cytokine(s) other than IL-7.

CD40-mediated regulation of interleukin-4 signaling pathways in B lymphocytes

The importance of cytokines in controlling immunoglobulin isotype switching is well known. Given the defect in switching to IgG, IgA and IgE isotypes in mice and humans that carry mutations in the CD40 and CD40 ligand genes, we have investigated the role of CD40 ligation in controlling B cell responses to interleukin (IL)-4. We have found that CD40-mediated signals cause a fivefold upregulation of IL-4 receptor (IL-4R) on the B cell surface and that this is associated with a 100-1000-fold increase in the cells' responsiveness to the cytokine. While we found no evidence of increased affinity or structural change of the receptor, we do find that prestimulation of B cells with anti-CD40 antibodies brings about several changes in the IL-4 signaling pathways. Subsequent delivery of IL-4 to CD40-prestimulated cells provokes intracellular signals distinct from those induced in resting B cells in response to IL-4. While resting B cells phosphorylate Jak3 kinase shortly after IL-4 activation, cells pre-incubated with anti-CD40 exhibit active dephosphorylation of this molecule and phosphorylation of proteins of around 45 kDa upon addition of IL-4. The common gamma chain, Jak3 and Jak1 can all be immunoprecipitated in normal amounts with the IL-4R chain after CD40 prestimulation. We show that the observed dephosphorylation of Jak3 may be due to a stable association with the src-homology protein tyrosine phosphatase SH-PTP2. In contrast, the enzyme appears to be inactive and to dissociate very quickly from the signaling complex in cells that are stimulated with IL-4 alone.

The cytoplasmic domain of syndecan-1 is required for cytoskeleton association but not detergent insolubility. Identification of essential cytoplasmic domain residues

Syndecan-1 is a member of a gene family of multifunctional transmembrane heparan sulfate proteoglycans that bind a variety of extracellular ligands and possess highly conserved non-catalytic cytoplasmic domains. It has been shown that antibody-mediated clustering of syndecan-1 causes the proteoglycan to become associated with microfilaments and insoluble in non-ionic detergent. A series of truncation and point mutations of the syndecan-1 core protein was constructed to identify specific structural features that were required for these characteristics. The transmembrane domain but not the cytoplasmic domain was required for cell surface expression of syndecan-1. Deletion of the COOH-terminal 11 amino acids of the cytoplasmic domain had no effect, while deletion of an additional 12 amino acids abolished microfilament association. Mutation of a conserved tyrosine residue within the latter region also abolished microfilament association. In contrast, mutation of 2 tyrosine residues outside this region had no effect. Deletion of the entire cytoplasmic domain (except for a short stop-transfer sequence) did not affect insolubility of the proteoglycan in detergent. Analysis of a form of syndecan-1 that lacked glycosaminoglycan acceptor sites revealed that covalently attached glycosaminoglycans were not required for cell surface expression, microfilament association, or detergent insolubility. These results demonstrate that microfilament association is a function of a subregion within the cytoplasmic domain and suggest that insolubility in detergent is a function of the transmembrane domain.

Interleukin-6 family of cytokines induced activation of different functional sites expressed by gp130 transducing protein

Gp130 transducing protein was shown to be involved in the formation of the high affinity receptors for interleukin 6 (IL-6), interleukin-11 (IL-11), leukemia inhibitory factor, oncostatin M (OSM), ciliary neurotrophic factor (CNTF), and cardiotrophin-1. In the present study we have characterized the functional properties of antibodies directed against this protein and identified a group of monoclonal antibodies able to antagonize the biological activities of all the cytokines belonging to the IL-6 cytokine family. The B-R3 pan-blocking antibody weakly interfered with the binding of the radiolabeled ligands (with the exception of OSM, whose binding was abrogated in the presence of B-R3 monoclonal antibody) but inhibited the gp130 homodimerization or its association with gp190/leukemia inhibitory factor receptor, as well as the subsequent tyrosine phosphorylation events. In addition we identified antibodies that were able to neutralize only one single cytokine of the IL-6 family. This was the case for the B-K5 antibody, which antagonized the binding of OSM to gp130 but did not interfere with the signals provided by the related cytokines triggering the proliferation of the TF1 erythroleukemia cell line or the induction of haptoglobin synthesis in the HepG2 hepatoma cell line. Similarly, we also characterized two additional antibodies B-P8 and B-P4, which inhibited the TF1 cell proliferation observed in the presence of CNTF and IL-11, respectively. B-P8 antibody only faintly interfered with the binding of the gp130-ligands and might modulate the signal transduction pathways. This study indicates that in addition to functional site(s) required by the whole family of IL-6 type cytokines to transduce the signal insight the cell, specific cognate functional sites were recruited by OSM, CNTF, or IL-11.

Expression of Janus kinase 3 in human endothelial and other non-lymphoid and non-myeloid cells

Members of the Janus kinase (Jak) family of protein tyrosine kinases have recently been implicated in the proximal signal transduction events of cytokine receptors. Jak3, a newly discovered member of this family, is believed to be normally limited in its expression to cells of the lymphoid and myeloid lineages. Herein we show that Jak3 is expressed in primary human vascular cells, as well as other non-lymphoid and non-myeloid cell types. Reverse transcriptase-polymerase chain reaction and Northern blot analysis revealed that Jak3 mRNA was expressed at low levels in human umbilical vein endothelial cells (HUVEC), human aortic smooth muscle cells (HASMC), A549 (human lung carcinoma), and DLD-1 (human colon adenocarcinoma) cells. Higher basal levels of Jak3 mRNA were detected in HMEC-1 (human microvascular cell line) and HepG2 (human hepatocellular carcinoma) cells. Jak3 mRNA expression was induced in HUVEC, HMEC-1, and HASMC by treatment with interleukin-1beta, tumor necrosis factor-alpha, interferon-gamma, and lipopolysaccharide. Jak3 protein was detectable at low levels in untreated HMEC-1, and these levels increased significantly with cytokine treatment. Furthermore, Jak3 protein was phosphorylated upon treatment of these cells with interleukin-4. This work shows that Jak3 is expressed or inducible in human vascular endothelial, vascular smooth muscle, and other non-lymphoid and non-myeloid cells, suggesting a broader role for Jak3 in the cytokine signal transduction of these cells.

Receptors for interleukin (IL)-10 and IL-6-type cytokines use similar signaling mechanisms for inducing transcription through IL-6 response elements

The cytoplasmic domain of the receptor for interleukin 10 (IL-10R) contains two box 3 sequence motifs that have been identified in the signal-transducing receptor subunits for IL-6-type cytokines and noted to be required for activating STAT3 and inducing transcription through IL-6-responsive elements. To determine whether the IL-10R has signaling functions similar to IL-6R in cells normally expressing these receptors, leukocytes of the B-, T-, and NK-cell lineages were treated with either cytokine. Both cytokines activated factors that bound to the sis-inducible element and included STAT1 and STAT3. The cell response to IL-10 characteristically differed from that to IL-2/IL-15, IL-4, and interferon gamma. The signaling capabilities of the IL-10R for activating specific STAT proteins and inducing gene transcription were defined by reconstitution of receptor functions in transfected tissue culture cells. COS-1 cells, co-expressing the human IL-10R and individual STAT proteins, confirmed a preference of the IL-10R for STAT3 and STAT1. Unlike many hematopoietin receptors, the IL-10R did not detectably activate STAT5. The IL-10R, together with reporter gene constructs containing different IL-6-responsive gene elements, reconstituted in hepatoma cells an induction of transcription by IL-10 that was comparable to that by IL-6. This regulation could not be appreciably modified by enhanced expression of STAT proteins. The similar actions of IL-10R and IL-6R on the induction of endogenous IL-6-responsive genes were demonstrated in hepatoma cells stably expressing the IL-10R. These receptor functions required the presence of the box 3 motifs, as shown by the analysis of the mouse IL-10R constructs containing progressively truncated cytoplasmic domains. The data demonstrate that the IL-10R, unlike other members of the interferon receptor family, is highly effective in recruiting the signaling pathways of IL-6-type cytokine receptors.

Ceramide signalling and the immune response

Ceramide, produced through either the induction of SM hydrolysis or synthesized de novo transduces signals mediating differentiation, growth, growth arrest, apoptosis, cytokine biosynthesis and secretion, and a variety of other cellular functions. A generalized ceramide signal transduction scheme is shown in Fig. 2 in which ceramide is generated through the activation of distinct SMases residing in separate subcellular compartments in response to specific stimuli. Clearly, specificity of cellular responses to ceramide depends upon many factors which include the nature of the stimulus, co-stimulatory signals and the cell type involved. Ceramide derived from neutral SMase activation is thought to be involved in modulating CAPK and MAP kinases, PLA2 (arachidonic acid mobilization), and CAPP while ceramide generated through acid SMase activation appears to be primarily involved in NF-kappa B activation. While there is no apparent cross-talk between these two ceramide-mediated signalling pathways, there is likely to be significant cross-talk between ceramide signalling and other signal transduction pathways (e.g., the PKC and MAP kinase pathways). Other downstream targets for ceramide action include Cox, IL-6 and IL-2 gene expression, PKC zeta, Vav, Rb, c-Myc, c-Fos, c-Jun and other transcriptional regulators. Many, if not all, of these ceramide-mediated signalling events have been identified in the various cells comprising the immune system and are integral to the optimal functioning of the immune system. Although the role of the SM pathway and the generation of ceramide in T and B lymphocytes have only recently been recognized, it is clear from these studies that signal transduction through SM and ceramide can strongly affect the immune response, either directly through cell signalling events, or indirectly through cytokines produced by other cells as the result of signalling through the SM pathway. An overview of the signalling mechanisms coupling ceramide to the modulation of the immune response is depicted in Fig. 3 and shows how ceramide may play pivotal roles in regulating a number of complex processes. The SM pathway represents a potentially valuable focal point for therapeutic control of immune responses, perhaps for either enhancement of the activity of T cells in the elimination of tumors, or the down-regulation of lymphocyte function in instances of autoimmune disease. The recent explosion of knowledge regarding ceramide signalling notwithstanding, a number of critical questions need to be answered before a comprehensive, mechanistic understanding can be formulated relative to the incredibly varied effects of ceramide on cell function. For example, (i) how is a structurally simple molecule like ceramide able to mediate so many different, and sometimes paradoxical, physiological responses ranging from cell proliferation and differentiation to inhibition of cell growth and apoptosis, (ii) what are the molecular identities and modes of activation of the various SMase isoforms, (iii) what determines the distribution of the unique isoforms of SMase in cells of different lineages or at different stages of differentiation, (iv) what is the relative contribution of ceramide generated through SM hydrolysis versus de novo synthesis, and (v) by what means does ceramide interact with specific intracellular targets? Although a number of ceramide-activatable kinases, phosphatases, and their protein substrates have been identified, a more extensive search for additional cellular targets will be indispensable in determining the phosphorylation cascades linking the activation of the SM pathway to the regulation of nuclear events. Clearly, cross-talk between ceramide-induced signal transduction cascades and other signalling pathways adds to the inherent difficulty in distinguishing the specific effects of complex, intertwining signalling pathways.

In vitro correction of JAK3-deficient severe combined immunodeficiency by retroviral-mediated gene transduction

Mutations affecting the expression of the Janus family kinase JAK3 were recently shown to be responsible for autosomal recessive severe combined immunodeficiency (SCID). JAK3-deficient patients present with a clinical phenotype virtually indistinguishable from boys affected by X-linked SCID, a disease caused by genetic defects of the common gamma chain (gamma c) that is a shared component of the receptors for IL-2, IL-4, IL-7, IL-9, and IL-15. The specific interaction of JAK3 and gamma c represents the biochemical basis for the similarities between these two immunodeficiencies. Both forms of SCID are characterized by recurrent, severe infections leading to death in infancy unless successfully treated by allogeneic bone marrow transplantation. Because of the potentially lethal complications associated with allogeneic bone marrow transplantation and the frequent lack of suitable marrow donors, the development of alternative forms of therapy is highly desirable. To this end, we investigated a retroviral-mediated gene correction approach for JAK3-deficiency. A vector carrying a copy of JAK3 cDNA was constructed and used to transduce B cell lines derived from patients with JAK3-deficient SCID. We demonstrate restoration of JAK3 expression and phosphorylation upon IL-2 and IL-4 stimulation. Furthermore, patients' cells transduced with JAK3 acquired the ability to proliferate normally in response to IL-2. These data indicate that the biological defects of JAK3-deficient cells can be efficiently corrected in vitro by retroviral-mediated gene transfer, thus providing the basis for future investigation of gene therapy as treatment for JAK3-deficient SCID.

ISGF3 gamma p48, a specificity switch for interferon activated transcription factors

Interferon (IFN) induces gene expression by phosphorylating latent transcription factors of the STAT family. Two different STAT multimeric complexes that bind distinct enhancer elements are activated by IFN alpha and IFN gamma, dictated by the DNA-binding protein ISGF3 gamma p48. This protein, a member of the IFN regulatory factor (IFR) family, acts as an adaptor protein to redirect STAT multimers from their intrinsic palindromic sequence specificity to interactions with a composite element composed of an IRF site juxtaposed with a STAT half-site. Sequence similarity within the IRF family suggests that other members could serve as adaptor proteins for transcriptional activators. Recent evidence suggests that PIP (LSIRF) sequesters the Ets protein PU.1 at a composite DNA element lends support to this adaptor hypothesis.

Ligand-induced dimerization of growth factor receptors: variations on the theme

Growth, differentiation, apoptosis and movement of cells are regulated, in part, by polypeptide growth factors, or cytokines, which exert their effects by binding to cell surface receptors on the target cells. Recent observations have indicated that a common mechanism of activation of several classes of such receptors is ligand-induced dimerization or oligomerization of the receptors.

Cloning and chromosomal mapping of bovine interleukin-2 receptor gamma gene

Interleukin-2 receptor (IL-2R) gamma chain, a member of the cytokine receptor superfamily, forms a high-affinity receptor with IL-2R alpha and beta chains that plays an important role in interleukin-2 (IL-2) signal transduction. We have cloned and characterized the bovine IL-2Rgamma gene and corresponding cDNA. Bovine IL-2Rgamma is a single-copy gene that contains 8 exons and spans approximately 3.8 kb. The promoter region lacks conventional TATA and CCAAT consensus sites, but contains several regulatory elements that are recognition sites for the GATA binding proteins, AP-1 and AP-2. Physical assignment by fluorescence in situ hybridization (FISH) placed the bovine IL-2Rgamma gene on chromosome Xq23.

Activation of Raf-mitogen-activated protein kinase signaling pathway by 1,25-dihydroxyvitamin D3 in normal human keratinocytes

The biologic effects of the vitamin D hormone 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) are believed to be mediated by an intracellular vitamin D receptor, which after ligand binding acts as a transcription factor modulating expression of a variety of genes. Besides having a well-known role in calcium metabolism, this hormone is an important regulator of proliferation in a majority of normal and neoplastic cells. Keratinocytes provide a convenient model for investigating the growth-related effects of vitamin D in normal cells. Growth of keratinocytes may be either stimulated or inhibited by 1,25(OH)2D3, depending on the degree of cell differentiation. We show here that 1,25(OH)2D3 stimulates DNA synthesis via sequential activation of Raf and the mitogen-activated protein kinase. Activation of these kinases is independent on protein and mRNA synthesis and is preceded by rapid tyrosine phosphorylation of an adaptor protein p66 (Shc) and formation of a complex between p66 Shc, a bridging molecule Grb2, and a Ras activator, mSos. Vitamin D receptor protein associates with Shc, indicating that this steroid hormone is able to signal through the transcription-independent pathways similar to those used by peptide hormones and cytokines.

Dispensability of Jak1 tyrosine kinase for interleukin-2-induced cell growth signaling in a human T cell line

The tyrosine kinases Jak1 and Jak3 are known to be associated with the beta and gamma chains of interleukin-2 receptor (IL-2R). They are activated by stimulation with IL-2, IL-4, IL-7, IL-9, or IL-15, receptors of which share the gamma chain of the IL-2R. We have obtained direct evidence of Jak1 association with the alpha chains of receptors for IL-4, IL-7 and IL-9 and with the beta chain of IL-2R, which is also common to the IL-15R. Furthermore, we have prepared mutant IL-2R beta chains with a mutation in the box 1 region, which is conserved among the IL-2R beta chain and the alpha chains of the other cytokine receptors sharing the IL-2R gamma chain. Using MOLT-4 transfectants with the mutant beta chains, we found that two conserved proline residues within the box 1 region are essentially involved in association with Jak1. The MOLT-4 transfectants with the mutant beta chains lacking Jak1 association showed IL-2 responsiveness, in terms of activation of Jak3 and Stat5 and induction of cell growth, indicating that Jak1 is dispensable for IL-2-mediated cell growth signaling and that Jak1 activation is not required for activation of Jak3 and Stat5 in the MOLT-4 transfectants.

STAT3beta, a splice variant of transcription factor STAT3, is a dominant negative regulator of transcription

The 89-kDa STAT3 protein is a latent transcription factor which is activated in response to cytokines (interleukin (IL)-5 and -6) and growth factors (epidermal growth factor). Binding of IL-5 to its specific receptor activates JAK2 which leads to the tyrosine phosphorylation of STAT3 proteins. Here we report the cloning of a cDNA encoding a variant of the transcription factor STAT3 (named STAT3beta) which was isolated by screening an eosinophil cDNA library. Compared to wild-type STAT3, STAT3beta lacks an internal domain of 50 base pairs located near the C terminus. This splice product is a naturally occurring isoform of STAT3 and encodes a 80-kDa protein. We found by reconstitution of the human IL-5R in COS cells that like STAT3, STAT3beta is phosphorylated on tyrosine and binds to the pIRE from the ICAM-1 promoter after IL-5 stimulation. However, STAT3beta fails to activate a pIRE containing promoter in transient transfection assays. Instead, co-expression of STAT3beta inhibits the transactivation potential of STAT3. These results suggests that STAT3beta functions as a negative regulator of transcription.

Evidence for a role for the phosphotyrosine-binding domain of Shc in interleukin 2 signaling

Stimulation via the T-cell growth factor interleukin 2 (IL-2) leads to tyrosine phosphorylation of Shc, the interaction of Shc with Grb2, and the Ras GTP/GDP exchange factor, mSOS. Shc also coprecipitates with the IL-2 receptor (IL-2R), and therefore, may link IL-2R to Ras activation. We have further characterized the Shc-IL-2R interaction and have made the following observations. (i) Among the two phosphotyrosine-interaction domains present in Shc, the phosphotyrosine-binding (PTB) domain, rather than its SH2 domain, interacts with the tyrosine-phosphorylated IL-2R beta chain. Moreover, the Shc-PTB domain binds a phosphopeptide derived from the IL-2R beta chain (corresponding to residues surrounding Y338, SCFTNQGpYFF) with high affinity. (ii) In vivo, mutant IL-2R beta chains lacking the acidic region of IL-2Rbeta (which contains Y338) fail to phosphorylate Shc. Furthermore, when wild type or mutant Shc proteins that lack the PTB domain were expressed in the IL-2-dependent CTLL-20 cell line, an intact Shc-PTB domain was required for Shc phosphorylation by the IL-2R, which provides further support for a Shc-PTB-IL-2R interaction in vivo. (iii) PTB and SH2 domains of Shc associate with different proteins in IL-2- and T-cell-receptor-stimulated lysates, suggesting that Shc, through the concurrent use of its two different phosphotyrosine-binding domains, could assemble multiple protein complexes. Taken together, our in vivo and in vitro observations suggest that the PTB domain of Shc interacts with Y338 of the IL-2R and provide evidence for a functional role for the Shc-PTB domain in IL-2 signaling.

JAK2 is essential for activation of c-fos and c-myc promoters and cell proliferation through the human granulocyte-macrophage colony-stimulating factor receptor in BA/F3 cells

Interleukin-3 (IL-3) or granulocyte-macrophage colony-stimulating factor (GM-CSF) is known to activate JAK2 in various cells, but the role of JAK2 in IL-3 or GM-CSF receptor signal transduction is largely unknown. We have now examined the role of JAK2 in GM-CSF-induced signaling events in BA/F3 cells. In BA/F3 cells expressing hGMR, activation of JAK2 by hGM-CSF requires the box1 region of hGMR beta. Dominant negative JAK2 (delta JAK2), which lacked the kinase domain suppressed mIL-3 or hGM-CSF-induced c-fos promoter activation as well as c-myc promoter activation/cell proliferation, thereby suggesting that JAK2 is involved in the signaling of both pathways. Further analyses of the role of JAK2 in c-fos gene activation in BA/F3 cells expressing hGMR revealed that delta JAK2 inhibited hGM-CSF-induced phosphorylation of Shc and protein tyrosine phosphatase 1D. Within hGMR beta, the several tyrosine residues which exist are related to activation of Shc or protein tyrosine phosphate 1D, and are phosphorylated in response to hGM-CSF stimulation. In addition, we observed that delta JAK2 inhibited hGM-CSF-induced phosphorylation of hGMR beta. Taken together, our results suggest that JAK2 activated by the box1 region of hGMR mediates hGM-CSF-induced c-fos promoter activation through phosphorylation of hGMR.

Characterization of the receptor binding sites of human leukemia inhibitory factor and creation of antagonists

Residues in human leukemia inhibitory factor (hLIF) crucial for binding to both the human LIF receptor (R) and gp130 were identified by analysis of alanine scanning mutants of hLIF in assays for both receptor binding and bioactivity. The region of hLIF most important for binding to the hLIF-R is composed of residues from the amino terminus of the D-helix, carboxyl terminus of the B-helix, and C-D loop. This site forms a distinct surface at the end of the four-helix bundle in the tertiary structure of the closely related murine LIF. The two residues of hLIF that contribute the majority of free energy for hLIF-R binding, Phe-156 and Lys-159 are surrounded by other residues which have only a moderate impact. This arrangement of a few key residues surrounded by less important ones is analogous to the functional binding epitope of human growth hormone for its receptor. A second region of hLIF that includes residues from the carboxyl terminus of the D-helix and A-B loop also had a weak influence on hLIF-R binding. Residues in hLIF from both the A- and C-helices are involved in binding the gp130 co-receptor. Abolition of the gp130 binding site in hLIF created antagonists of LIF action.

gp130 transducing receptor cross-linking is sufficient to induce interleukin-6 type responses

gP130 transducing receptor is involved in the formation of high affinity receptors for the cytokines of the interleukin-6 (IL-6) family. Recruitment of gp130 by IL-6 associated to its receptor leads to the dimerization of the transducing component. In the present study we did characterize the B-S12 monoclonal antibody raised against gp130 and able to elicit IL-6 type biological activities. B-S12 antibody triggered strongly the proliferation of TF1 and XGI hematopoietic cell lines and was able to increase the synthesis of acute phase proteins in HepG2 hepatoma cell line. B-S12 also behaved as a synergistic factor with granulocyte-macrophage colony-stimulating factor for both proliferation and differentiation of CD34-positive hematopoietic cell progenitors. By using a symmetric enzyme-linked immunosorbent assay, allowing the detection of dimeric forms of soluble gp130, we found that addition of B-S12 to gp130 led to its dimerization. Analysis of the tyrosine phosphorylation events in gp130 and Jak kinase family members revealed that B-S12 quickly induced the phosphorylation of gp130 in a neural derived cell line, and that Jak1 and Jak2 were also recruited. In conclusion, we show that gp130 cross-linking with the B-S12 monoclonal antibody was sufficient to generate functional IL-6 type responses in hematopoietic, neural, and hepatic cells.

Thrombopoietin potentiates activation of human platelets in association with JAK2 and TYK2 phosphorylation

Thrombopoietin (TPO), also known as the c-mpl ligand, stimulates rapid tyrosine phosphorylation of multiple proteins in human platelets including the Janus family kinases JAK2 and TYK2. On its own, TPO has no effect on platelet aggregation and dense-granule secretion but induces a general potentiation of these responses by other stimuli. The most dramatic effect is observed against threshold concentrations of agonists for aggregation. Shape change or weak reversible aggregation induced by low concentrations of thrombin, collagen and the thromboxane mimetic, U46619, are converted into irreversible aggregation in the presence of TPO. A similar result is obtained in the presence of the ADP scavenger apyrase and cyclo-oxygenase inhibitor indomethacin. TPO also induces potentiation of dense-granule secretion measured through release of 5-hydroxy[3H]-tryptamine. This effect is most striking against low concentrations of stimuli and is independent of aggregation as it is observed in the presence of chelation of extracellular Ca2+ with EGTA. TPO potentiates activation of phospholipase C and elevation of intracellular Ca2+, providing a molecular explanation for potentiation of functional responses. TPO may have an important physiological role in priming platelet activation in thrombocytopenia, an action that may help to compensate for the reduced platelet density.

Molecular characterization of an alpha interferon receptor 1 subunit (IFNaR1) domain required for TYK2 binding and signal transduction

Binding of alpha interferon (IFNalpha) to its receptors induces rapid tyrosine phosphorylation of the receptor subunits IFNaR1 and IFNaR2, the TYK2 and JAK1 tyrosine kinases, and the Stat1 and Stat2 transcription factors. Previous studies have demonstrated that TYK2 directly and specifically binds to and tyrosine phosphorylates IFNaR1 in vitro. We now report a detailed analysis of the TYK2 binding domain on the IFNaR1 subunit. First, we used an in vitro binding assay to identify the TYK2 binding motif in IFNaR1 as well as the critical residues within this region. The most striking feature is the importance of a number of hydrophobic and acidic residues. A minor role is also ascribed to a region resembling the proline-rich "box 1" sequence. In addition, mutations which disrupt in vitro binding also disrupt the coimmunoprecipitation of the receptor and TYK2. We also provide direct evidence that the binding region is both necessary and sufficient to activate TYK2 in vivo. Specifically, mutations in the binding domain act in a dominant-negative fashion to inhibit the IFNalpha-induced tyrosine phosphorylation of TYK2 and Stat2. Further, introduction of dimerized glutathione S-transferase-IFNaR1 fusion proteins into permeabilized cells is sufficient to induce phosphorylation of TYK2 and the receptor, confirming the role of the binding domain in IFNalpha signal transduction. These studies provide clues to the sequences determining the specificity of the association between JAK family tyrosine kinases and cytokine receptors as well as the functional role of these kinases in cytokine signal transduction.

A novel STAT-like factor mediates lipopolysaccharide, interleukin 1 (IL-1), and IL-6 signaling and recognizes a gamma interferon activation site-like element in the IL1B gene

Binding of many cytokines to their cognate receptors immediately activates Jak tyrosine kinases and their substrates, STAT (signal transducers and activators of transcription) DNA-binding proteins. The DNA binding targets of STATs are sequence elements related to the archetypal gamma interferon activation site, GAS. However, association of interleukin 1 (IL-1) with Jak-STAT signaling has remained unresolved. We now report an element termed LILRE (lipopolysaccharide [LPS] and IL-1-responsive element) in the human prointerleukin 1beta gene (IL1B) which can be immediately induced by either lipopolysaccharide (LPS) or IL-1 protein to bind a tyrosine-phosphorylated protein. This LPS- and IL-1-induced factor (LIL factor) is recognized by an antibody raised against the N terminus of Stat1, but not by those specific for either the C terminus of Stat1 or any other GAS-binding STAT. Phosphotyrosine (P-Tyr) specifically inhibits formation of the LIL factor-DNA complex, suggesting the importance of P-Tyr for the DNA-binding activity, as has been found for all STAT dimers. Analysis of DNA-binding specificity demonstrates that the LIL factor possesses a novel GAS-like binding activity that contrasts with those of other STATs in a requirement for a G residue at position 8 (TTCCTGAGA). Further investigation has revealed that IL-6, but neither IL-4 nor gamma interferon, activates the LIL factor. Thus, the existence of such a STAT-like factor (LIL-Stat) relates the LPS and IL-1 signaling pathway to other cytokine receptor signaling pathways via the activation of STATs. Moreover, the unique DNA-binding specificity and antigenicity of this factor suggest that LPS, IL-1, and IL-6 may use a common signaling pathway.

Multiple cis-trans conformers of the prolactin receptor proline-rich motif (PRM) peptide detected by reverse-phase HPLC, CD and NMR spectroscopy

An eight-amino-acid synthetic peptide (IIe1-Phe2-Pro3-Pro4-Val5-Pro6-Gly7-Pro8) corresponding to the conserved proline-rich motif (PRM) of the intracellular domain of the prolactin receptor (PRL-R) was studied by one- and two-dimensional (1D and 2D) proton NMR spectroscopy in water and DMSO in order to characterize its conformational dynamics. The purified PRL-R PRM peptide eluted as two partially resolved peaks in equilibrium on reverse-phase HPLC (RP-HPLC) at 20 degrees C with a ratio of 60:40. At 30 degrees C, the two peaks coalesced into a single peak The two RP-HPLC peaks correspond to two peptide conformers resulting from the slow cis-trans isomerization of one of the four proline amide bonds. Although the peptide has only three amide (NH) protons, its ID NMR spectrum in water contains approximately 15 discernible NH region peaks, providing evidence for multiple conformers. The amide resonances were assigned on the basis of 2D-COSY spectra, chemical shift values resonance splitting patterns and temperature coefficients. The cis:trans ratio for each proline in water, calculated from integrated intensities and/or peak heights of the appropriate resonances, were Phe2-Pro3 (35:65), Pro3-Pro4 (40:60), Val5-Pro6 (70:30), and Gly7-Pro8 (30:70). Temperature studies (25-70 degrees C) were used to semi-quantitatively estimate the rates of isomerization for the different prolines. In water, Pro8 undergoes rapid isomerization; Pro3 isomerizes at an intermediate rate; while Pro4 and Pro6 both appear to isomerize very slowly since no coalescence of amide resonances was observed. In DMSO, only Pro4 displayed slow isomerization. Slow kinetics combined with a similar 60:40 ratio of conformers determined by RP-HPLC and NMR suggests that isomerization of the Pro3-Pro4 bond generates the two RP-HPLC peaks. Both proximal and distal proline isomerization effects were observed in NMR experiments. All of the 16 theoretical (24 = 16) proline configurations appear to exist in equilibrium in water The predominant (19%) conformation, trans3-trans4-cis6-trans8, may reflect the configuration of the PRM prolines in the native PRL-R. Isomerization of Pro6 from cis to trans generates an interaction between the peptide N-and C-termini, suggesting an overall pseudo-cyclic conformation. This all-trans proline configuration may play an important biochemical role in the function of cytokine/haematopoietin receptors. A model is proposed which suggests that isomerization of the PRM by an immunophilin such as the FK 506-binding protein (FKBP) serves as an on-off switch for cytokine receptor activation.

STAT3 activation is a critical step in gp130-mediated terminal differentiation and growth arrest of a myeloid cell line

Myeloid leukemia M1 cells can be induced for growth arrest and terminal differentiation into macrophages in response to interleukin 6 (IL-6) or leukemia inhibitory factor (LIF). Recently, a large number of cytokines and growth factors have been shown to activate the Janus kinase (JAK)-signal transducer and activator of transcription (STAT) signaling pathway. In the case of IL-6 and LIF, which share a signal transducing receptor gp130, STAT3 is specifically tyrosine-phosphorylated and activated by stimulation with each cytokine in various cell types. To know the role of JAK-STAT pathway in M1 differentiation, we have constructed dominant negative forms of STAT3 and established M1 cell lines that constitutively express them. These M1 cells that overexpressed dominant negative forms showed no induction of differentiation-associated markers including Fc gamma receptors, ferritin light chain, and lysozyme after treatment with IL-6. Expression of either c-myb or c-myc was not downregulated. Furthermore, IL-6- and LIF-mediated growth arrest and apoptosis were completely blocked. Thus these findings demonstrate that STAT3 activation is the critical step in a cascade of events that leads to terminal differentiation of M1 cells.

Delta-9-tetrahydrocannabinol: an inhibitor of STAT1 alpha protein tyrosine phosphorylation

Tyrosine-phosphorylated signal transducer and activator of transcription 1 alpha (STAT1 alpha) is a 91-kDa protein responsible for interferon-gamma (IFN-gamma)-dependent transcription. The present study demonstrates that activation by IFN-gamma of murine macrophages resulted in tyrosine phosphorylation of STAT1 alpha identified by immunoprecipitation. The tyrosine phosphorylation of STAT1 alpha was found highly sensitive to treatment by delta-9 tetrahydrocannabinol (THC), a major marijuana component. Subsequently, the isoform formation of p91 due to tyrosine phosphorylation was reduced in THC-treated macrophages. Although inhibition by THC of the tyrosine phosphorylation of STAT1 alpha induced by IFN-gamma was in a THC concentration-related manner, the tyrosine phosphorylation of other proteins induced by lipopolysaccharide/IFN-gamma treatment of macrophages appeared insensitive to THC treatment. Our data suggest that blockade by THC of tyrosine phosphorylation of STAT1 alpha may be an important mechanism involved in the broad immunosuppressive effects of THC.

Multiple growth factors are associated with lesions of atherosclerosis: specificity or redundancy?

Within the last five years, a number of specific growth factors have been localized in developing lesions of atherosclerosis. This localization of growth factors that is not observed in normal vessels, together with the pleotrophic activities of growth factors, have suggested a role for growth factors in atherosclerotic lesion progression. However, based on in vitro studies, many of the growth factors identified in lesions have overlapping target cells and are derived from the same cellular sources. What is the relative role of the specific growth factors identified? How is the their activity altered by the local conditions in the vessel wall? How do different risk factors for atherosclerosis alter the balance between growth factors and their natural regulators? Evidence for the involvement of specific growth factors in the progression of lesions of atherosclerosis is discussed, as well as the multiple levels at which the activities of these growth factors may be regulated by the vessel wall.

Platelet-derived growth factor induces phosphorylation of multiple JAK family kinases and STAT proteins

Receptors for interferons and other cytokines signal through the action of associated protein tyrosine kinases of the JAK family and latent cytoplasmic transcription factors of the STAT family. Genetic and biochemical analysis of interferon signaling indicates that activation of STATs by interferons requires two distinct JAK family kinases. Loss of either of the required JAKs prevents activation of the other JAK and extinguishes STAT activation. These observations suggest that JAKs provide interferon receptors with a critical catalytic signaling function and that at least two JAKs must be incorporated into an active receptor complex. JAK and STAT proteins are also activated by ligands such as platelet-derived growth factor (PDGF), which act through receptors that possess intrinsic protein tyrosine kinase activity, raising questions about the role of JAKs in signal transduction by this class of receptors. Here, we show that all three of the ubiquitously expressed JAKs--JAK1, JAK2, and Tyk2--become phosphorylated on tyrosine in both mouse BALB/c 3T3 cells and human fibroblasts engineered to express the PDGF-beta receptor. All three proteins are also associated with the activated receptor. Through the use of cell lines each lacking an individual JAK, we find that in contrast to interferon signaling, PDGF-induced JAK phosphorylation and activation of STAT1 and STAT3 is independent of the presence of any other single JAK but does require receptor tyrosine kinase activity. These results suggests that the mechanism of JAK activation and JAK function in signaling differs between receptor tyrosine kinases and interferon receptors.

Genetic analysis of tyrosine kinase function in B cell development

B lymphopoiesis is regulated by multiple signals from stromal cell contact, soluble cytokines, antigen, and T helper cells. In vitro and biochemical experiments have implicated tyrosine kinases as key components of many of these signaling pathways. Genetic analysis of the role of these tyrosine kinases has been facilitated by recent advance in transgenic and gene targeting technology as well as by the identification of the genetic basis of several human and murine immune deficiencies. This review discusses the effect of gain and loss of function mutations of selected tyrosine kinases and their regulators and substrates on B cell development and function.

Activation and association of Stat3 with Src in v-Src-transformed cell lines

STAT proteins are a group of latent cytoplasmic transcription factors which function as signal transducers and activators of transcription. Stat1 and -2 were originally identified to function in interferon signaling, and Stat1 was also found to be activated by epidermal growth factor (EGF) and other cytokines. New members of the STAT gene family are identified. Among them, Stat3 has 52.5% amino acid sequence homology with Stat1 and is activated by platelet-derived growth factor (PDGF), colony-stimulating factor 1 (CSF-1), EGF, interleukin-6, and other cytokines. Treatment of cells with EGF activates Stat1 and Stat3, which become phosphorylated on tyrosine residues to form homo - or heterodimers and translocate into the nucleus, binding to the sis-inducible element (SIE) in the c-fos promoter. Somatic cell genetic analyses demonstrated that Jaks, a family of nontransmembrane protein tyrosine kinases, are required for the activation of Stat1 and Stat2 in interferon-treated cells. However, little is known about the activation of Stat3 by growth factors. Here we report that in all v-Src-transformed cell lines examined, Stat3 is constitutively activated to bind to DNA and the phosphorylation of tyrosine on Stat3 is enhanced by the induction of v-Src expression. We also report that Src is shown to be associated with Stat3 in vivo, as well as in vitro, and phosphorylates Stat3 in vitro. Stat3 is also activated by CSF-1, possibly through CSF-1 receptor-c Src association in NIH 3T3 cells overexpressing CSF-1 receptors. Together, the data suggest that Src is involved in activation of Stat3 in growth factor signal transduction.

The interleukin-2 receptor gamma chain: its role in the multiple cytokine receptor complexes and T cell development in XSCID

Interleukin 2 (IL-2), a T cell-derived cytokine, targets a variety of cells to induce their growth, differentiation, and functional activation. IL-2 inserts signals into the cells through IL-2 receptors expressed on cell surfaces to induce such actions. In humans, the functional IL-2 receptor consists of the subunit complexes of the alpha, beta and gamma chains, or the beta and gamma chains. The third component, the gamma chain, of IL-2 receptor plays a pivotal role in formation of the full-fledged IL-2 receptor, together with the beta chain, the gamma chain participates in increasing the IL-2 binding affinity and intracellular signal transduction. Moreover, the cytokine receptors for at least IL-2, IL-4, IL-7, IL-9, and IL-15 utilize the same gamma chain as an essential subunit. Interestingly, mutations of the gamma chain gene cause human X-linked severe combined immunodeficiency (XSCID) characterized by a complete or profound T cell defect. Among the cytokines sharing the gamma chain, at least IL-7 is essentially involved in early T cell development in the mouse organ culture system. The molecular identification of the gamma chain brought a grasp of the structures and functions of the cytokine receptor and an in-depth understanding of the cause of human XSCID. To investigate the mechanism of XSCID and development of gene therapy for XSCID, knockout mice for the gamma chain gene were produced that showed similar but not exactly the same phenotypes as human XSCID.

Different interleukin 2 receptor beta-chain tyrosines couple to at least two signaling pathways and synergistically mediate interleukin 2-induced proliferation

One of the earliest events induced by interleukin 2 (IL-2) is tyrosine phosphorylation of cellular proteins, including the IL-2 receptor beta chain (IL-2Rbeta). Simultaneous mutation of three tyrosines (Y338, Y392, and Y510) in the IL-2Rbeta cytoplasmic domain abrogated IL-2-induced proliferation, whereas mutation of only Y338 or of Y392 and Y510 inhibited proliferation only partially. While Y392 and Y510 were critical for IL-2-induced activation of signal transducers and activators of transcription (STAT proteins), Y338 was required for Shc-IL-2Rbeta association and for IL-2-induced tyrosine phosphorylation of Shc. Thus, activation of both Jak-STAT and Shc-coupled signaling pathways requires specific IL-2Rbeta tyrosines that together act in concert to mediate maximal proliferation. In COS-7 cells, overexpression of Jak1 augmented phosphorylation of Y338 as well as Y392 and Y510, suggesting that the role for this Jak kinase may extend beyond the Jak-STAT pathway.

Mechanisms of interleukin-1beta regulation of nitric oxide synthase in cardiac myocytes

Cytokines and endotoxin stimulate inducible NO synthase (iNOS) in different types of cells; however, little is known about regulatory mechanisms. Using the Griess reagent for nitric levels, Western blots for iNOS protein, Northern blots for iNOS mRNA, and transient transfection studies to monitor transcription, we determined potential mechanisms involved in interleukin-1beta stimulation of iNOS in cultured neonatal ventricular myocytes. When myocytes were treated with interleukin-1beta (5 ng/mL), nitrite levels increased, and this effect was inhibited 80% by the specific iNOS inhibitor aminoguanidine. Neither interferon gamma nor tumor necrosis factor-alpha alone stimulated nitrite production. Bacterial endotoxin alone stimulated nitrites and potentiated the effect of interleukin. To determine whether a tyrosine kinase-mediated signaling pathway was involved in interleukin action, we used the inhibitor genistein, which blocked interleukin-stimulated nitrites, iNOS protein, and iNOS mRNA. To determine the effect of activation of protein kinase C, we treated cells with the phorbol ester phorbol 12-myristate 13-acetate (PMA). PMA decreased both interleukin-stimulated nitrites and iNOS protein by 40%. To determine the involvement of cyclic nucleotides, cells were treated with either dibutyryl cAMP or cGMP. cAMP (1 mmol/L) stimulated iNOS mRNA, protein, and nitrite production, whereas cGMP had no effect. To test for a direct effect of interleukin on transcription of the iNOS gene, we transfected the full-length mouse iNOS 5' regulatory sequences (-1592 to +160) coupled to a luciferase reporter gene (-1592iNOSLuc). Interleukin stimulated luciferase activity 1.8 +/- 0.2-fold. To determine whether interleukin also affects iNOS mRNA stability, interleukin-stimulated iNOS mRNA was allowed to decay in the presence of the transcription inhibitor actinomycin D. iNOS mRNA t1/2 (approximately 1 hour) was not affected by interleukin. Thus, our data suggest that (1) interleukin-1beta is the primary cytokine in myocyte iNOS regulation and acts predominantly at the transcriptional level; (2) interleukin stimulation of iNOS mRNA and protein is coupled to a tyrosine kinase-mediated signaling pathway; and (3) protein kinase C and cAMP can modify interleukin signaling by decreasing and increasing iNOS, respectively.

Association of Lyn tyrosine kinase with the nuclear matrix and cell-cycle-dependent changes in matrix-associated tyrosine kinase activity

The nuclear matrix isolated from HeLa cells and Rat2 fibroblasts harbors tyrosine kinase and tyrosine phosphatase activities. Polypeptides of 53, 56 and 60 kDa, associated with this subnuclear structure, were phosphorylated at tyrosine in vivo. By immunoblot and immunolabelling experiments, we identified one of the nuclear-matrix-associated tyrosine kinases as Lyn, a Src family member. Lyn was distributed as foci throughout the matrix. The p56 and p53 isoforms of Lyn remained firmly associated with the nuclear matrix after a variety of matrix preparation procedures, and were not detectable in the chromatin fraction of the nucleus. The tyrosine kinase activity associated with the nuclear matrix showed cell-cycle-dependent changes, maximum activity being observed at the G1/S transition phase. Polyoma-virus-transformed rat fibroblast cells showed sixfold higher tyrosine kinase activity in the nuclear matrix preparations compared to that in untransformed cells. These observations are consistent with the suggestion that tyrosine kinase activity associated with the nuclear matrix may be an important determinant of cellular proliferation.

Marelle acts downstream of the Drosophila HOP/JAK kinase and encodes a protein similar to the mammalian STATs

We have identified a putative Drosophila STAT protein named Marelle that exhibits mutant phenotypes identical to mutations in the Hopscotch/JAK kinase. We show that a reduction in the amount of marelle gene activity suppresses the phenotype associated with a gain-of-function mutation in hopscotch and enhances the phenotype associated with a weak hopscotch mutation. We propose that Hopscotch activates Marelle to regulate transcription of target genes such as the pair rule gene even-skipped. Our results demonstrate the existence of an invertebrate JAK/STAT system.

The molecular basis of X-linked severe combined immunodeficiency: defective cytokine receptor signaling

X-linked severe combined immunodeficiency (XSCID) is an inherited disease characterized by profoundly diminished cell-mediated and humoral immunity. XSCID was found to result from mutations in the interleukin-2 (IL-2) receptor gamma chain. Knowledge of the genetic defect has important implications for prenatal and postnatal diagnosis, carrier female identification, and the possibility of gene therapy. The fact that the phenotype and clinical manifestations in XSCID are more severe than the abnormalities found in humans or mice deficient in IL-2 led to the speculation and subsequent confirmation that the IL-2 receptor is not the only receptor to contain the gamma chain. Instead, the gamma chain is also a component of the receptors for IL-4, IL-7, IL-9, and IL-15 and is now denoted as the common cytokine receptor gamma chain, gamma c. The role of gamma c in signaling and lymphoid development and the implications of a shared receptor component are discussed.

Evolution of the src-related protein tyrosine kinases

A phylogenetic analysis of src-related protein tyrosine kinases (PTKs) showed that one group of these genes is quite ancient in the animals, its divergence predating the divergence of the diploblast and triploblast phyla. Three other major groupings of genes were found to predate the divergence of protostome and deuterostome phyla. Most known src-related PTKs of mammals were found to belong to five well-differentiated families: srcA, srcB, abl, csk, and tec. One srcA gene (fyn) has an alternatively spliced seventh exon which shows a different pattern of relationship from the remainder of the gene; this suggests that this exon may have been derived by a recombinational event with another gene, perhaps one related to fgr. The recently published claim that mammalian members of this family expressed in the nervous system evolve more slowly at nonsynonymous nucleotide sites than do those expressed in the immune system was not supported by an analysis of 13 pairs of human and mouse orthologues. Rather, T-cell-specific src-related PTKs were found to have higher rates of nonsynonymous substitution than were those having broader expression. This effect was particularly marked in the peptide binding site of the SH2 domain. While the SH2 binding site was highly conserved among paralogous mammalian members of the srcA and srcB subfamilies, no such effect was seen in the comparison of paralogous members of the csk and tec subfamilies. This suggests that, while the peptide binding function of SH2 is conserved within both srcA and srcB subfamilies, paralogous members of the csk and tec subfamilies have diverged functionally with respect to peptide recognition by SH2.