Complementation by the protein tyrosine kinase JAK2 of a mutant cell line defective in the interferon-gamma signal transduction pathway

Fyn associates with Cbl and phosphorylates tyrosine 731 in Cbl, a binding site for phosphatidylinositol 3-kinase

We have investigated the interaction between Cbl and the Src-related tyrosine kinase Fyn. Fyn was observed to be constitutively associated with Cbl in lysates of several different cell types including the interleukin-3-dependent murine myeloid cell line 32Dcl3, and the prolactin-dependent rat thymoma cell line Nb2. Binding studies indicated that Cbl could bind to glutathione S-transferase (GST) fusion proteins encoding the unique, Src homology domain 3 (SH3), and SH2 domains of Fyn, Hck, or Lyn. Fusion proteins encoding either the SH3 or SH2 domains of Fyn bound to Cbl as effectively as the fusion protein encoding the unique, SH3, and SH2 domains of Fyn. The Fyn SH2 domain bound to both tyrosine-phosphorylated and nonphosphorylated Cbl, implying that this interaction might be phosphotyrosine-independent. Binding of the Fyn SH2 domain to Cbl was not disrupted by the addition of phosphotyrosine, phosphoserine, or phosphothreonine. A GST fusion protein encoding the proline-rich region of Cbl bound to Fyn present in a total cell lysate. Far Western blot analysis also indicated that the SH3 domain of Fyn bound preferentially to the proline-rich region of Cbl. The addition of [gamma-32P]ATP to either anti-Cbl immunoprecipitates or anti-Fyn immunoprecipitates resulted in the phosphorylation of both Cbl and Fyn as demonstrated by immunoprecipitation of the phosphorylated proteins with specific antisera. Fyn directly phosphorylated a GST fusion protein containing the C-terminal region of Cbl (GST-CBL-LZIP). In contrast, immunoprecipitated JAK2 was not able to phosphorylate this same region of Cbl. The GST-CBL-LZIP fusion protein contains a binding site for the SH2 domain of the p85 subunit of phosphatidylinositol 3-kinase, which mapped to Tyr731, which is present in the sequence YEAM. Mutation of Tyr731 in GST-CBL-LZIP eliminated binding of the p85 subunit of phosphatidylinositol 3-kinase and substantially reduced the phosphorylation of this fusion protein by Fyn, despite the presence of four other tyrosine residues in this fusion protein. These data are consistent with the hypothesis that Cbl represents a substrate for Src-like kinases that are activated in response to the engagement of cell surface receptors, and that Src-like kinases are responsible for the phosphorylation of a tyrosine residue in Cbl that may regulate activation of phosphatidylinositol 3-kinase.

The Jak-STAT pathway: cytokine signalling from the receptor to the nucleus

The Jak-STAT pathway was originally discovered through the study of interferon induced intracellular signal transduction. Meanwhile, a large number of cytokines, hormones and growth factors have been found to activate Jaks and STATs. Jaks (Janus Kinases) are a unique class of tyrosine kinases that associate with cytokine receptors. Upon ligand binding, they activate members of the Signal Transducers and Activators of Transcription (STAT) family through phosphorylation on a single tyrosine. Activated STATs form dimers, translocate to the nucleus, bind to specific response elements in promotors of target genes, and transcriptionally activate these genes. Both positive and negative regulations of the Jak-STAT pathway have been identified. In a positive feedback loop, interferons transcriptionally activate the genes for components of the interferon stimulated gene factor 3 (ISGF3). A number of cytokines that activate the Jak-STAT pathway, e.g. IL-6, IL-4, LIF, G-CSF, have been shown to upregulate the expression of SOCS-JABs-SSIs, a recently discovered class of STAT inhibitors. Targeted disruption of genes for a number of Jaks and STATs in mice have revealed specific biological functions for many of them. Although most of the STATs are activated in cell culture by many different ligands, STAT knockout mice mostly show defects in a single or a few cytokine dependent processes. STAT1 knockout mice have an impaired interferon signalling, STAT4 knockouts impaired IL-12 signalling, STAT5a knockouts impaired prolactin signalling, STAT5b knockouts impaired growth hormone signalling, and STAT6 knockout impaired IL-4 and IL-13 signalling. Defects in the Jak-STAT pathway have already been identified in a number of human diseases. Prominent amongst them are leukaemias, lymphomas and inherited immunodeficiency syndromes. It can be expected that additional Jak-STAT related diseases will be identified over the next years. To date, specific STAT inhibitory drugs are not known, but a number of specific protein-protein interactions in the Jak-STAT pathway are potential targets for pharmaceutical interventions.

Jeg-3 human choriocarcinoma-induced immunosuppression: downregulation of interleukin-2, interleukin-2 receptor alpha-chain, and its Jak/Stat signaling pathway

PROBLEM

The mechanisms of the immunosuppressive and immunosuppression-inducing capacities of Jeg-3 human choriocarcinoma cell line supernatants (HCSs) are not yet completely understood. The influence on interleukin (IL)-2, IL-4 and interferon (IFN)-gamma production; IL-2 receptor (IL-2R) alpha-, beta-, and gamma-chain; and the signaling pathway molecules Janus kinase (Jak)1, Jak3, signal transducers and activators of transcription (Stat)1, Stat3, and Stat5 should be investigated.

METHOD OF STUDY

For assessment of IL production, whole peripheral venous blood from healthy donors was stimulated with phorbol-myristate-acetate and ionomycine. Secretion of ILs was blocked with monensine. Intracellular ILs were analyzed by flow cytometry. For IL-2R and signaling pathway molecule analysis, peripheral blood lymphocytes were stimulated with phytohemagglutinin (PHA). IL-2R chains were measured by flow cytometry, and Jaks/Stats by sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) and Western blot.

RESULTS

Phorbol-myristate-acetate and ionomycine strongly increase the percent-age of IL-2+ cells; an additional 50% HCSs significantly suppresses the percentage to, or below the level of unstimulated cells. IFN-gamma production is strongly decreased by HCSs in some cases, but not in others. PHA stimulates IL-2R alpha-, beta-, and gamma-chain expression and their signaling pathway molecules Jak1, Jak3, Stat1, Stat3, and Stat5. 50% HCS downregulates the alpha-chain and slightly upregulates the beta-chain. Jak1, Jak3, Stat1, Stat3, and Stat5 expression is suppressed approximately to, or below the level of unstimulated cells.

CONCLUSIONS

HCS forcefully blocks the production of IL-2; the IL-2R alpha-chain; and Jak1, Jak3, Stat1, Stat3, and Stat5 expression. The observed phenomena might be caused by downregulation of an IL-2R regulation gene, and might play a key role in the expansion of choriocarcinoma, and possibly in the survival of the fetal allograft.

The suppressor of cytokine signaling (SOCS) 1 and SOCS3 but not SOCS2 proteins inhibit interferon-mediated antiviral and antiproliferative activities

The suppressor of cytokine signaling (SOCS) proteins are a family of cytokine-inducible negative regulators of cytokine signaling. Interferon (IFN)-gamma treatment induces the expression of SOCS1, SOCS2, and SOCS3 mRNAs. To examine the effect of SOCS proteins on IFN-mediated Janus-activated kinase/signal transducers and activators of transcription (STAT) signaling, HeLa- and MCF-7-derived stable cell lines expressing SOCS1, SOCS2, and SOCS3 proteins were established. SOCS1 and SOCS3 but not SOCS2 inhibited the tyrosine phosphorylation and nuclear translocation of STAT1 in response to IFN stimulation. The IFN-mediated antiviral and antiproliferative activities were consistently blocked by the constitutive expression of SOCS1 and SOCS3 but not SOCS2 proteins. The maximum inhibitory activities of SOCS1 and SOCS3 proteins toward the activation of STAT1 were observed at very low levels of SOCS protein expression. In addition, SOCS1 exhibited a much stronger inhibitory activity toward the activation of STAT1 than did SOCS3. These results suggest that SOCS1 and SOCS3 but not SOCS2 are inhibitors of IFN-mediated Janus-activated kinase/STAT signaling pathways.

Requirement for Distinct Janus Kinases and STAT Proteins in T Cell Proliferation Versus IFN-γ Production Following IL-12 Stimulation

While IL-12 is known to activate JAK2 and TYK2 and induce the phosphorylation of STAT4 and STAT3, little is known regarding how the activation of these signaling molecules is related to the biologic effects of IL-12. Using an IL-12-responsive T cell clone (2D6), we investigated their requirements for proliferation and IFN-γ production of 2D6 cells. 2D6 cells could be maintained with either IL-12 or IL-2. 2D6 lines maintained with IL-12 (2D6IL-12) or IL-2 (2D6IL-2) exhibited comparable levels of proliferation, but produced large or only small amounts of IFN-γ, respectively, when restimulated with IL-12 after starvation of either cytokine. 2D6IL-12 induced TYK2 and STAT4 phosphorylation. In contrast, their phosphorylation was marginally induced in 2D6IL-2. The reduced STAT4 phosphorylation was due to a progressive decrease in the amount of STAT4 protein along with the passages in IL-2-containing medium. 2D6IL-12 and 2D6IL-2 similarly proliferating in response to IL-12 induced comparable levels of JAK2 activation and STAT5 phosphorylation. JAK2 was associated with STAT5, and IL-12-induced STAT5 phosphorylation was elicited in the absence of JAK3 activation. These results indicate that IL-12 has the capacity to induce/maintain STAT4 and STAT5 proteins, and that TYK2 and JAK2 activation correlate with STAT4 phosphorylation/IFN-γ induction and STAT5 phosphorylation/cellular proliferation, respectively.

Activation of the JAK-STAT pathway by reactive oxygen species

Reactive oxygen species (ROS) play an important role in the pathogenesis of many human diseases, including the acute respiratory distress syndrome, Parkinson's disease, pulmonary fibrosis, and Alzheimer's disease. In mammalian cells, several genes known to be induced during the immediate early response to growth factors, including the protooncogenes c-fos and c-myc, have also been shown to be induced by ROS. We show that members of the STAT family of transcription factors, including STAT1 and STAT3, are activated in fibroblasts and A-431 carcinoma cells in response to H2O2. This activation occurs within 5 min, can be inhibited by antioxidants, and does not require protein synthesis. STAT activation in these cell lines is oxidant specific and does not occur in response to superoxide- or nitric oxide-generating stimuli. Buthionine sulfoximine, which depletes intracellular glutathione, also activates the STAT pathway. Moreover, H2O2 stimulates the activity of the known STAT kinases JAK2 and TYK2. Activation of STATs by platelet-derived growth factor (PDGF) is significantly inhibited by N-acetyl-L-cysteine and diphenylene iodonium, indicating that ROS production contributes to STAT activation in response to PDGF. These findings indicate that the JAK-STAT pathway responds to intracellular ROS and that PDGF uses ROS as a second messenger to regulate STAT activation.

Role of the STAT1-SH2 domain and STAT2 in the activation and nuclear translocation of STAT1

Interferon (IFN) induction of immediate-early response genes is mediated through the signal transducers and activators of transcription (STATs). Activation of STAT1 by IFNalpha or IFNgamma through its tyrosine phosphorylation involves members of the Jak tyrosine kinases. In addition, STAT2 is activated by IFNalpha, and, together with STAT1 and p48/ISGF3gamma, forms the transcription factor complex ISGF3. Previous findings suggested that the STAT1-SH2 domain, which is required for the homo- or heterodimerization of STAT1, also participates in the recruitment of STAT1 to the IFN-receptors, because mutations in the SH2-domain abolished STAT1 activation by IFNgamma. Furthermore, STAT2 was reported to be required for the activation of STAT1 by IFNalpha. We were able to induce STAT1 tyrosine phosphorylation by IFNalpha/beta in the absence of STAT2 or a functional STAT1-SH2 domain. In contrast, IFNgamma was unable to cause tyrosine phosphorylation of STAT1-(SH2:Arg --> Gln). Interestingly, although STAT1 was found in the nucleus in STAT2-deficient cells, the nuclear accumulation of the tyrosine phosphorylated SH2-mutant STAT1 was impaired. In summary, our results indicate that the SH2 domain of STAT1 is not required for its ligand-dependent activation by IFNalpha/beta. Moreover, tyrosine phosphorylation is not sufficient to target STAT1 to the nucleus; rather, dimerization appears to play a critical role in the subcellular distribution of STAT1.

JAK/STAT-deficient cell lines

Mutant cell lines B3 and B10, which are unresponsive to both interferon (IFN)-alpha and IFN-gamma, and line B9, which does not respond to IFN-gamma stimulation, are described. The mutants were submitted to fluorescence-activated cell sorting (FACS) from a cellular pool, which was obtained from the parental cell line 2C4 after several rounds of mutagenesis. The unresponsiveness to IFN stimulation was observed both in terms of expression of cell surface markers (CD2, class I and II HLAs) and mRNA expression of IFN-stimulated genes (2'-5' oligoadenylate synthetase (OAS), 9-27, and guanylate binding protein (GBP)). Genetic crossing of B3, B9 and B10 with U3 (STAT1-), gamma 2a (JAK2-) and U4 (JAK1-) mutants, respectively, did not restore IFN responsiveness to the hybrid cell lines. However, when these cell lines were crossed with the same mutants, but using the pairwise crosses B3 x U4, B9 x U3 and B10 x U3, the cell hybrids recovered full IFN responsiveness. The present genetic experiments permitted us to assign the mutant cell lines B3, B9 and B10 to the U3, gamma 2 and U4 complementation groups, respectively. These conclusions were supported by the analysis of IFN-stimulated genes in the mutants.

Stat3 plays an important role in oncogenic Ros- and insulin-like growth factor I receptor-induced anchorage-independent growth

The role of signal transducers and activators of transcription (STATs) in receptor protein-tyrosine kinase (PTK)-induced cell growth and transformation was investigated using an inducible epidermal growth factor receptor-Ros chimeric receptor called ER2 and a constitutively activated insulin-like growth factor I receptor called NM1, both of which are able to induce anchorage-independent growth of NIH 3T3 cells. ER2 and NM1 receptor PTKs are able to cause Stat3 activation. Co-expressing the dominant negative Stat3 mutant with ER2 or NM1 in transiently or stable transfected cells resulted in a dramatic inhibition of colonies induced by these receptor PTKs and a moderate inhibition of their mitogenicity in monolayer. Therefore, Stat3 is not only important for initiation of transformation, as demonstrated by inhibition of the epidermal growth factor-inducible colony formation of the ER2 cells by the mutant, but it is also required for the maintenance of transformation, as evidenced by reversion of the NM1 transformed cells. The DNA binding and transcriptional activities of the endogenous Stat3 were greatly inhibited in the ER2 and NM1 cells co-expressing the Stat3 mutants. We conclude that activated function of Stat3 is required for the establishment and maintenance of Ros and insulin-like growth factor I receptor PTK-induced cell transformation.

Oligodendroglial survival factors, PDGF-AA and CNTF, activate similar JAK/STAT signaling pathways

Platelet-derived growth factor (PDGF) and ciliary neurotrophic factor (CNTF) have pleiotropic actions on many cell types. In the presence of these factors, oligodendroglia respond by enhanced survival when deprived of trophic factors or in the presence of the cytotoxic cytokine, tumor necrosis factor-alpha (TNF-alpha). To determine whether these two oligodendroglial survival factors converge in their signaling cascades, we examined their JAK/STAT pathways in enriched oligodendrocyte (OL) progenitors and in the progenitor OL cell line, central glia-4 (CG-4). Cytokine pathways such as JAK/STAT have been characterized extensively in hematopoietic cells; however, it is increasingly evident that the same cytokines that play a role in hematopoiesis also play a role during development and injury of the central nervous system. This is the first study that clearly defines the presence and activation of JAK/STAT proteins in OL progenitors and compares the signal transduction pathway of two well-known oligodendroglial survival factors. In this study, we report that PDGF- and CNTF-induced OL progenitors responded with a rapid tyrosine phosphorylation of JAK1, JAK2, STAT1alpha/beta, and STAT3. We feel that these identified JAK/STAT signaling molecules play a large role in the cellular response to these factors. Because both PDGF and CNTF enhance OL progenitor survival, these JAK/STATs may play a role in regulating this important cellular process.

Construction of a Lipopolysaccharide Reporter Cell Line and Its Use in Identifying Mutants Defective in Endotoxin, But Not TNF-α, Signal Transduction

Gram-negative bacterial LPS is a potent activator of inflammatory responses. The binding of LPS to CD14 initiates signal transduction; however, the molecular processes immediately following this event remain unclear. We engineered an LPS-inducible fibroblast reporter cell line to facilitate the use of molecular genetic techniques to study the LPS signaling pathway. A plasmid containing the human Tac Ag cDNA under transcriptional control of the human E selectin promoter was cotransfected into Chinese hamster ovary (CHO)-K1 cells together with a CD14 expression plasmid. A cell line was obtained, 3E10, which up-regulated expression of Tac following stimulation with LPS. Pools of mutagenized cells were exposed to LPS and then labeled with anti-Tac mAb. Cells that failed to up-regulate Tac expression were enriched by flow cytometry. Thirty clonal mutant cell lines were identified that continued to express CD14 and bind LPS, but failed to express Tac or translocate nuclear factor-κB (NF-κB) following LPS exposure. TNF-α-treated mutant cells continued to express Tac and translocate NF-κB. An analysis of LPS-induced NF-κB activity in heterokaryons derived from polyethylene glycol-fused cell lines indicated that recessive mutations in genes encoding components of the LPS signaling pathway accounted for the signaling defects. To date, two complementation groups have been identified from 11 cell lines analyzed. These data demonstrate that the TNF-α signaling pathway diverges from the LPS pathway early in the signal-transduction cascade despite similarities in LPS- and TNF-α-induced responses. Identification of the genes affected in these mutant reporter cells should identify heretofore-elusive components of the LPS signaling cascade.

A mutant cell line defective in response to double-stranded RNA and in regulating basal expression of interferon-stimulated genes

Although much progress has been made in identifying the signaling pathways that mediate the initial responses to interferons (IFNs), much less is known about how IFN-stimulated genes (ISGs) are kept quiescent in untreated cells, how the response is sustained after the initial induction, and how ISG expression is down-regulated, even in the continued presence of IFN. We have used the cell sorter to isolate mutant cells with constitutively high ISG expression. A recessive mutant, P2.1, has higher constitutive ISG levels than the parental U4C cells, which do not respond to any IFN. Unexpectedly, P2.1 cells also are deficient in the expression of ISGs in response to double-stranded RNA (dsRNA). Electrophoretic mobility-shift assays revealed that the defect is upstream of the activation of the transcription factors NFkappaB and IFN regulatory factor 1. Analysis of the pivotal dsRNA-dependent serine/threonine kinase PKR revealed that the wild-type kinase is present and is activated normally in response to dsRNA in P2.1 cells. Together, these data suggest that the defect in P2.1 cells is either downstream of PKR or in a component of a distinct pathway that is involved both in activating multiple transcription factors in response to dsRNA and in regulating the basal expression of ISGs.

Studies of IFN-induced transcriptional activation uncover the Jak-Stat pathway

Sharing the Milstein Award with George R. Stark and Ian M. Kerr in the fall of 1997 brought this invitation to record personal reflections on our experiments concerning the mechanisms of action of interferon. Our work and that of the Kerr and Stark laboratories uncovered the Jak-Stat pathway through which signals from cell surface receptors reach genes in the cell nucleus. This review concentrates on that to which I can speak most reliably, that is, experiments done by my young colleagues at Rockefeller University.

Targeted disruption of the interferon-gamma receptor 2 gene results in severe immune defects in mice

To study the role of the interferon- (IFN) gammaR2 chain in IFN-gamma signaling and immune function, IFN-gammaR2-deficient mice have been generated and characterized. Cells derived from IFN-gammaR2 -/- mice are unable to activate either JAK/STAT signaling proteins or gene transcription in response to IFN-gamma. The lack of IFN-gamma responsiveness alters IFN-gamma-induced Ig class switching by B cells from these mice. In vitro cultures of T cells demonstrate that the T cells from the IFN-gammaR2 -/- mice have a defect in Th1 cell differentiation. The IFN-gammaR2 (-/-) mice also produce lower amounts of IFN-gamma in response to antigenic challenge. In addition, IFN-gammaR2 -/- mice are defective in contact hypersensitivity and are highly susceptible to infection by Listeria monocytogenes. These results demonstrate that the IFN-gammaR2 is essential for IFN-gamma-mediated immune responses in vivo.

IFN-γ Priming Up-Regulates IFN-Stimulated Gene Factor 3 (ISGF3) Components, Augmenting Responsiveness of IFN-Resistant Melanoma Cells to Type I IFNs

IFN-stimulated gene factor 3 (ISGF3) mediates transcriptional activation of IFN-sensitive genes (ISGs). The component subunits of ISGF3, STAT1αβ, STAT2, and p48-ISGF3γ, are tyrosine phosphorylated before their assembly into a complex. Subsequently, the ISGF3 complex is translocated to the nucleus. We have recently established that the responsiveness of human melanoma cell lines to type I IFNs correlates directly with their intracellular levels of ISGF3 components, particularly STAT1. In the present study, we show that pretreating IFN-resistant melanoma cell lines with IFN-γ (IFN-γ priming) before stimulation with type I IFN also results in increased levels of ISGF3 components and enhanced DNA-binding activation of ISGF3. In addition, IFN-γ priming of IFN-resistant melanoma cell lines increased expression of type I IFN-induced ISG products, including ISG54, 2′-5′-oligoadenylate synthase, HLA class I, B7-1, and ICAM-1 Ags. Furthermore, IFN-γ priming enhanced the antiviral effect of IFN-β on the IFN-resistant melanoma cell line, MM96. These results support a role for IFN-γ priming in up-regulating ISGF3, thereby augmenting the responsiveness of IFN-resistant melanoma cell lines to type I IFN and providing a molecular basis and justification for using sequential IFN therapy, as proposed by others, to enhance the use of IFNs in the treatment of melanoma.

Modulation of interferon-gamma-induced macrophage activation by phosphotyrosine phosphatases inhibition. Effect on murine Leishmaniasis progression

Phagocyte functions are markedly inhibited after infection with the intracellular protozoan parasite Leishmania. This situation strongly favors the installation and propagation of this pathogen within its mammalian host. Previous findings by us and others have established that alteration of several signaling pathways (protein kinase C-, Ca2+- and protein-tyrosine kinases-dependent signaling events) were directly responsible for Leishmania-induced macrophage (MO) dysfunctions. Here we report that modulation of phosphotyrosine-dependent events with a protein tyrosine phosphatases (PTP) inhibitor, the peroxovanadium (pV) compound bpV(phen) (potassium bisperoxo(1,10-phenanthroline)oxovanadate(Vi)), can control host-pathogen interactions by different mechanisms. We observed that the inhibition of parasite PTP resulted in an arrest of proliferation and death of the latter in coincidence with cyclin-dependent kinase (CDK1) tyrosine 15 phosphorylation. Moreover the treatment of MO with bpV(phen) resulted in an increased sensitivity to interferon-gamma stimulation, which was reflected by enhanced nitric oxide (NO) production. This enhanced IFN-gamma-induced NO generation was accompanied by a marked increase of inducible nitric oxide synthase (iNOS) mRNA gene and protein expression. Finally we have verified the in vivo potency of bpV(phen) over a 6-week period of daily administration of a sub-toxic dose. The results revealed its effectiveness in controlling the progression of visceral and cutaneous leishmaniasis. Therefore PTP inhibition of Leishmania and MO by the pV compound bpV(phen) can differentially affect these eukaryotic cells. This strongly suggests that PTP plays an important role in the progression of Leishmania infection and pathogenesis. The apparent potency of pV compounds along with their relatively simple and versatile structure render them attractive pharmacological agents for the management of parasitic infections.

Jaks and STATs: biological implications

Cytokines and interferons are molecules that play central roles in the regulation of a wide array of cellular functions in the lympho-hematopoietic system. These factors stimulate proliferation, differentiation, and survival signals, as well as specialized functions in host resistance to pathogens. Although cytokines are known to activate multiple signaling pathways that together mediate these important functions, one of these pathways, the Jak-STAT pathway, is the focus of this chapter. This pathway is triggered by both cytokines and interferons, and it very rapidly allows the transduction of an extracellular signal into the nucleus. The pathway uses a novel mechanism in which cytosolic latent transcription factors, known as signal transducers and activators of transcription (STATs), are tyrosine phosphorylated by Janus family tyrosine kinases (Jaks), allowing STAT protein dimerization and nuclear translocation. STATs then can modulate the expression of target genes. The basic biology of this system, including the range of known Jaks and STATs, is discussed, as are the defects in animals and humans lacking some of these signaling molecules.

Mechanisms of cytokine signal transduction: IL-2, IL-4 and prolactin as hematopoietin receptor models

Cytokines, hormones and hematopoietic growth factors transduce biological signals across the cell membrane via a highly conserved family of single membrane-spanning receptors. The intracellular signal transducing machinery responsible for mediating these responses has remained largely unknown. However, recent identification of a homologous class of tyrosine kinases, Janus Kinases (JAKs), and a related family of transcription factors, signal transducers and activators of transcription (STATs), has shed new light on the molecular mechanisms responsible for mediating hematopoietin signaling and immune response. Current research efforts within the field of cytokine signaling have now shifted to understanding how these molecules are activated by hematopoietic receptors, positively and negatively regulated by kinases and phosphatases, and how they impact on gene transcription to ultimately coordinate cell homeostasis, proliferation and differentiation. This article will review some of our results identifying the involvement of JAKs, STATs, and secondary effector molecules activated following engagement of hematopoietic receptors for IL-2, IL-4, and prolactin. Here, we provide evidence for the ingenious ability of cytokine receptors to selectively recruit and activate these proteins among a repertoire of possible alternative biochemical messengers as a means to affect unique and general cell responses.

Jak2 deficiency defines an essential developmental checkpoint in definitive hematopoiesis

Janus kinases (Jaks) play an important role in signal transduction via cytokine and growth factor receptors. 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.

Analysis of the IFN-γ-Signaling Pathway in Macrophages at Different Stages of Maturation

We previously demonstrated that the macrophage cell lines RAW 264.7 and WEHI-3 exhibit distinct patterns of gene expression in response to IFN-γ. This difference is controlled at the transcriptional level and results from a specific inability of the less mature WEHI-3 cells to utilize either the IFN-stimulated response element or the γ-activated sequence DNA regulatory element in response to stimulation with IFN-γ, while other aspects of IFN-γ gene induction remain intact. In the work described here, we examined the components of the IFN-γ signal transduction pathway in RAW 264.7 and WEHI-3 cells to determine whether differences in pathway components or activity exist in WEHI-3 cells that could give rise to this difference in transcriptional response. Reverse transcriptase-PCR (RT-PCR) and flow cytometric analyses indicated that the levels of IFN-γ receptor mRNA accumulation and protein expression are comparable for RAW 264.7 and WEHI-3 cells. RT-PCR and immunoblot analyses revealed that the principal components of this signaling pathway, including JAK1, JAK2, and STAT1, are present in both RAW 264.7 and WEHI-3 cells. However, analysis of STAT1 DNA-binding activity by electrophoretic mobility shift assay and of STAT1 phosphorylation by immunoblot revealed that this DNA-binding factor is active in RAW 264.7, but not in WEHI-3, cells after IFN-γ stimulation. These results demonstrate that the components of the IFN-γ signal transduction pathway are intact in WEHI-3 cells, but stimulation of these cells by IFN-γ does not result in STAT1 activation.

Growth control mechanisms in multiple myeloma

Interleukin-6 (IL-6) is the major growth factor for the malignant plasma cell clone in patients with multiple myeloma (MM). Although interferon-alpha (IFN-alpha) has been widely used as maintenance therapy in MM, controversy exists as to its clinical utility. This review summarizes data showing that cell growth arrest brought about by type I (IFNs-alpha/beta) and type II (IFN-gamma) IFNs occurs in part through utilization/modification of various components of the otherwise stimulatory Jak-STAT and Ras signaling pathways triggered by IL-6. Recent experimental results indicating that IFN-alpha acts as a survival factor for certain myeloma cell lines and frequently induces endogenous IL-6 expression may help to explain the conflicting clinical findings obtained in this heterogeneous disease with this usually potent growth inhibitor. By comparison, consistent antiproliferative activity exhibited by IFN-gamma on IL-6-dependent myeloma cell lines and primary myeloma cells from patients suggests that further investigation of the possible value of this cytokine in the treatment of MM may be warranted.

Activation of the signal transducer gp130 by interleukin-11 and interleukin-6 is mediated by similar molecular interactions

The transmembrane glycoprotein gp130 is involved in many cytokine-mediated cellular responses and acts therein as the signal transducing receptor subunit. Interleukin-6 (IL-6) and interleukin-11 (IL-11), in complex with their specific alpha-receptors, homodimerize gp130 and, as a consequence, activate the Janus kinase (Jak)/signal transducer and activator of transcription (STAT) signalling pathway in their target cells. So far, it is not clear whether gp130 is bound to these cytokines and their specific alpha-receptor subunits through identical or different epitopes. In order to study the interaction of IL-11 and IL-11R with human gp130 the soluble form of the recently cloned human IL-11R was expressed in baculovirus-infected insect cells. By a coprecipitation binding-assay it is demonstrated that IL-11 and IL-6 compete for binding to gp130. Using deletion and point mutants of gp130 it is shown that IL-11-IL-11R and IL-6-IL-6R recognize overlapping binding motifs on gp130. Moreover, using well-established Jak-deficient cell lines we demonstrate that STAT activation by IL-11 requires Jak1. Taken together, our data support the concept that IL-6 and IL-11 activate gp130 by very similar molecular mechanisms.

Disruption of the Jak1 gene demonstrates obligatory and nonredundant roles of the Jaks in cytokine-induced biologic responses

Herein we report the generation of mice lacking the ubiquitously expressed Janus kinase, Jak1. Jak1-/- mice 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 depend on the gp130 subunit for signaling. Our 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.

Jak2 is essential for signaling through a variety of cytokine receptors

A variety of cytokines activate receptor-associated members of the Janus family of protein tyrosine kinases (Jaks). To assess the role of Jak2, we have derived Jak2-deficient mice. The mutation causes an embryonic lethality due to the absence of definitive erythropoiesis. Fetal liver myeloid progenitors, although 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 failed to respond to interferon gamma (IFNgamma), although the responses to IFNalpha/beta and IL-6 were unaffected. Lastly, 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.

Prostaglandin receptors in the kidney: a new route for intervention?

Prostaglandins comprise a structurally diverse family of cyclooxygenase metabolites of arachidonic acid, including thromboxane A2, PGD2, PGE2, PGF2alpha and PGI2. These prostaglandins are now known to act via different G-protein-coupled receptors. PGE2, the major prostaglandin synthesized along the nephron, interacts with at least four E-prostanoid (EP) receptors, three of which are highly expressed in distinct regions of the kidney. Each EP receptor also preferentially couples to a different signal transduction pathway, including stimulation of cAMP generation by the EP2 and EP4 receptors; inhibition of cAMP generation, via Gi by EP3 receptors, and activation of phosphatidylinositol hydrolysis by EP1 receptor. Other intrarenal prostanoid receptors include the thromboxane A2 receptor and the prostacyclin receptor. These receptors also exhibit a discrete intrarenal distribution. The possibility of pharmacologically targeting each renal prostanoid receptor may provide a unique approach to modifying renal function in disease states.

Selection and analysis of a mutant cell line defective in the hypoxia-inducible factor-1 alpha-subunit (HIF-1alpha). Characterization of hif-1alpha-dependent and -independent hypoxia-inducible gene expression

Hypoxia-inducible expression has been demonstrated for many groups of mammalian genes, and studies of transcriptional control have revealed the existence of hypoxia-responsive elements (HREs) in the cis-acting sequences of several of these genes. These sequences generally contain one or more binding sites for a heterodimeric DNA binding complex termed hypoxia-inducible factor-1 (HIF-1). To analyze this response further, Chinese hamster ovary cells were stably transfected with plasmids bearing HREs linked to genes encoding immunoselectable cell surface markers, and clones that showed reduced or absent hypoxia-inducible marker expression were selected from a mutagenized culture of cells. Analysis of these cells revealed several clones with transacting defects in HRE activation, and in one the defect was identified as a failure to express the alpha-subunit of HIF-1. Comparison of hypoxia-inducible gene expression in wild type, HIF-1alpha-defective, and HIF-1alpha-complemented cells revealed two types of response. For some genes (e.g. glucose transporter-1), hypoxia-inducible expression was critically dependent on HIF-1alpha, whereas for other genes (e.g. heme oxygenase-1) hypoxia-inducible expression appeared largely independent of the expression of HIF-1alpha. These experiments show the utility of mutagenesis and selection of mutant cells in the analysis of mammalian transcriptional responses to hypoxia and demonstrate the operation of HIF-1alpha-dependent and HIF-1alpha-independent pathways of hypoxia-inducible gene expression in Chinese hamster ovary cells.

Growth hormone regulation of SIRP and SHP-2 tyrosyl phosphorylation and association

SIRPs (signal-regulatory proteins) are a family of transmembrane glycoproteins that were identified by their association with the Src homology 2 domain-containing protein-tyrosine phosphatase SHP-2 in response to insulin. Here we examine whether SIRPalpha and SHP-2 are signaling molecules for the receptors for growth hormone (GH), leukemia inhibitory factor (LIF), or interferon-gamma (IFNgamma), cytokine receptor superfamily members that bind to and activate Janus kinase 2 (JAK2). In 3T3-F442A fibroblasts, GH rapidly stimulates tyrosyl phosphorylation of both SIRPalpha and SHP-2 and enhances association of SHP-2 with SIRPalpha. Consistent with JAK2 binding and phosphorylating SIRPalpha in response to GH, co-expression of SIRPalpha and JAK2 in COS cells results in tyrosyl phosphorylation of SIRPalpha and JAK2 association with SIRPalpha. LIF does not stimulate tyrosyl phosphorylation of SIRPalpha but stimulates greater tyrosyl phosphorylation of SHP-2 than GH. Additionally, LIF enhances association of SHP-2 with the gp130 subunit of the LIF receptor signaling complex. IFNgamma, which stimulates JAK2 to a greater extent than LIF, is ineffective at stimulating tyrosyl phosphorylation of SIRPalpha or SHP-2. These results suggest that SIRPalpha is a signaling molecule for GH but not for LIF or IFNgamma. Differential phosphorylation of SIRPalpha and SHP-2 may contribute to the distinct physiological effects of these ligands.

In vitro interaction between STAT 5 and JAK 2; dependence upon phosphorylation status of STAT 5 and JAK 2

A working model for haematopoietic cytokine signal transduction has been hypothesised as follows. Binding of cytokines to specific receptor molecules leads to phosphorylation and activation of receptor associated members of the Janus kinase family. This is followed by tyrosine phosphorylation of the associated receptor and members of the STAT (signal transducer and activator of transcription) family of DNA-binding transcription factors. Phosphorylation is accompanied by STAT dimerisation, nuclear transport and activation of gene transcription. Activation of gene transcription is mediated by the binding of STAT dimers to palindromic STAT response elements. A number of areas of confusion remain; not least the mechanism by which multiple cytokines signal via a limited number of STATs. A role has been suggested for phosphorylated receptor tyrosine residues as STAT docking sites on activated receptor-JAK complexes. According to this model the amino acid sequence context of key tyrosine residues confers receptor specificity upon STAT activation. There is some controversy as to whether this model applies to STAT 5. The heterologous expression of STAT 5 in Sf 9 insect cells using the baculovirus expression system is described here. Protein of the correct molecular weight was expressed and found to be phosphorylated on tyrosine residues and to bind to a STAT response DNA element. This binding was dependent upon the phosphorylation status of the STAT protein. DNA binding could be abolished in vitro by treatment with a phosphotyrosine phosphatase and restored in vitro by treatment with activated recombinant JAK 2. The protein was purified to near homogeneity using a simple ion exchange/gel filtration chromatography procedure. The interaction between purified recombinant STAT 5 and JAK 2, either expressed by baculovirus or endogenously expressed in Buffalo rat liver cells, was studied. In both cases STAT 5 in its non-phosphorylated form was found to form a stable complex with activated JAK 2. Non-activated JAK 2 and phosphorylated STAT 5 were unable to participate in complex formation. The results presented provide a mechanistic basis for the activation of STAT 5 by a wide range of cytokines capable of activating JAK 2.

Human cytomegalovirus inhibits major histocompatibility complex class II expression by disruption of the Jak/Stat pathway

Human cytomegalovirus (HCMV) is a ubiquitous herpesvirus that is able to persist for decades in its host. HCMV has evolved protean countermeasures for anti-HCMV cellular immunity that facilitate establishment of persistence. Recently it has been shown that HCMV inhibits interferon gamma (IFN-gamma)-stimulated MHC class II expression, but the mechanism for this effect is unknown. IFN-gamma signal transduction (Jak/Stat pathway) and class II transactivator (CIITA) are required components for IFN-gamma-stimulated MHC class II expression. In this study, we demonstrate that both a clinical isolate and a laboratory strain of HCMV inhibit inducible MHC class II expression at the cell surface and at RNA level in human endothelial cells and fibroblasts. Moreover, reverse transcriptase polymerase chain reaction and Northern blot analyses demonstrate that neither CIITA nor interferon regulatory factor 1 are upregulated in infected cells. Electrophoretic mobility shift assays reveal a defect in IFN-gamma signal transduction, which was shown by immunoprecipitation to be associated with a striking decrease in Janus kinase 1 (Jak1) levels. Proteasome inhibitor studies with carboxybenzyl-leucyl-leucyl-leucine vinyl sulfone suggest an HCMV-associated enhancement of Jak1 protein degradation. This is the first report of a mechanism for the HCMV-mediated disruption of inducible MHC class II expression and a direct virus-associated alteration in Janus kinase levels. These findings are yet another example of the diverse mechanisms by which HCMV avoids immunosurveillance and establishes persistence.

IFN-gamma induces calcium transients and increases the capacitative calcium entry in human neutrophils

We have previously reported that long-term priming of human polymorphonuclear neutrophilic granulocytes (PMN) with interferon-gamma (IFN-gamma) increased the fMLP-stimulated calcium influx. We now show that also after short-term incubation with IFN-gamma, PMN calcium metabolism is modulated. Single adherent cells in three different calcium-containing buffers (high, normal, and low [Ca2+]) were stimulated with the bacterial peptide fMLP or the Ca-ATPase inhibitor thapsigargin (Tg) after about 5 min preincubation with IFN-gamma. The results of this protocol indicated that IFN-gamma increases both calcium influx and calcium sequestration. Store dependent Ca2+ influx, directly measured on readdition of calcium to Tg-treated cells incubated in EGTA buffer, was significantly enhanced in IFN-gamma-treated cells. This effect of IFN-gamma was enhanced by the tyrosine kinase inhibitor herbimycin A. Strikingly, in low extracellular calcium concentrations, IFN-gamma induced calcium transients in 20%-60% of the cells. The proportion of PMN responding with Ca2+ transients increased with decreasing extracellular calcium concentration. Average lagtime from addition of IFN-gamma to a response that could be measured was 7.3 sec, and average increase in [Ca2+] above the basal level was 790 nM. These IFN-gamma-induced transients could not be depressed by herbimycin A. Thus, IFN-gamma can increase capacitative calcium influx, induce calcium transients, and possibly affect calcium sequestration in human PMN.

Human IFN gamma receptor cytoplasmic domain: expression and interaction with HuIFN gamma

To investigate the structural basis for human interferon gamma (huIFN gamma) binding to intracellular regions of the human IFN gamma receptor (huIFN gamma R), we have subcloned and expressed the huIFN gamma R free of fusion proteins in the yeast strain Pichia pastoris. HuIFN gamma bound to the cytoplasmic domain of the receptor via the IFN gamma C-terminus. Binding was inhibited by both human and mouse C-terminus peptides. N-terminus peptides failed to inhibit cytoplasmic binding. Thus, while extracellular receptor domain binding is species specific, binding to the cytoplasmic domain of the receptor is species non-specific. In solid-phase binding assays, IFN gamma had a Kd of 3.7 x 10(-8) M for the newly expressed cytoplasmic domain. Peptide competitions showed that IFN gamma bound to a receptor site corresponding to the membrane proximal residues 253-287, which is adjacent to the site of binding of the tyrosine kinase JAK2. The cytoplasmic binding affinity and binding site specificity suggest that the huIFN gamma R cytoplasmic domain can function independent of the extracellular domain to bind huIFN gamma and induce the biological activity previously associated with internalized huIFN gamma.

Effect of propranolol and IFN-beta on the induction of MHC class II expression and cytokine production by IFN-gamma IN THP-1 human monocytic cells

This study was undertaken to investigate the effects of propranolol, IFN-beta, and the protein kinase modulators on IFN-gamma induction of MHC class II antigen expression and cytokine production in THP-1 human monocytic cells. IFN-gamma induced expression of HLA-DR and DQ molecules and secretion of the monokines IL-1 beta and TNF-alpha in THP-1 cells in a time and dose-dependent manner. The effect of INF-gamma on class II HLA antigens was dose-dependently inhibited by IFN-beta. H-7, phloretin, staurosporine as well as GF 109203X are selective enzyme inhibitors of protein kinase C (PKC), down-regulating IFN-gamma induced MHC class II expression and cytokine production. Stimulators of PKC, like PMA, replaced IFN-gamma in the induction of monokines in THP-1 cells, whereas the addition of HA 1004 or arachidonic acid to the culture had no effect on IFN-gamma mediated changes. Blocking of phospholipase D (PLD)-derived diacylglycerol (DAG) formation by propranolol abrogated IFN-gamma increased HLA class II expression and IL-1 beta secretion, but had little effect on IFN-gamma induced TNF-alpha production. These findings appear to suggest that PLD-derived phosphatidate is not the primary source of DAG production in IFN-gamma-induced TNF-alpha secretion, but may be necessary for IFN-gamma-mediated MHC class II induction and IL-1 beta production in human monocytes, whereas phospholipase A2 may not be required for IFN-gamma activation of PKC in the process.

Cloning and Characterization of the Human Homolog of Mouse Jak2

Members of the Jak family play a critical role in signal transduction mediated by cytokine and hormone receptors. In this study, we report the cloning and characterization of human Jak2. The predicted amino acid sequence shows 91% homology to the described murine Jak2, but with a significant difference in the extreme C-terminal sequence. Using the human cDNA as a probe, we localized the gene for human Jak2 to chromosome 9p23-24. Human Jak2 mRNA is highly expressed in the spleen, lymph nodes, and peripheral blood lymphocytes (PBLs). A polyclonal antibody raised against the unique C-terminus of human Jak2 was used to characterize Jak2 protein. Levels of Jak2 protein expression increased significantly in mitogen- and anti-IgM–stimulated B cells and to a lesser degree in activated T cells. In addition, high levels of Jak2 protein were detected in pre-B leukemia cells.

Cloning and Characterization of the Human Homolog of Mouse Jak2

Members of the Jak family play a critical role in signal transduction mediated by cytokine and hormone receptors. In this study, we report the cloning and characterization of human Jak2. The predicted amino acid sequence shows 91% homology to the described murine Jak2, but with a significant difference in the extreme C-terminal sequence. Using the human cDNA as a probe, we localized the gene for human Jak2 to chromosome 9p23-24. Human Jak2 mRNA is highly expressed in the spleen, lymph nodes, and peripheral blood lymphocytes (PBLs). A polyclonal antibody raised against the unique C-terminus of human Jak2 was used to characterize Jak2 protein. Levels of Jak2 protein expression increased significantly in mitogen- and anti-IgM–stimulated B cells and to a lesser degree in activated T cells. In addition, high levels of Jak2 protein were detected in pre-B leukemia cells.

Suppression of Cell Proliferation and the Expression of abcr-abl Fusion Gene and Apoptotic Cell Death in a New Human Chronic Myelogenous Leukemia Cell Line, KT-1, by Interferon-α

A new human leukemia cell line, KT-1, was established from a patient in the blastic crisis phase of chronic myelogenous leukemia (CML). This cell line had a positive reaction for intracytoplasmic myeloperoxidase and two Philadelphia chromosomes (Ph1) [t(9;22)(q34;q11)] and lacked normal copies of chromosomes 9 and 22. Molecular characterization of the breakpoint in the t(9;22)(q34;q11) showed that KT-1 had a bcr-2/abl-2 splice junction. When the KT-1 cells were cultured with interferon (IFN)-α or IFN-γ, the growth of the cells were dose-dependently suppressed. IFN-α and IFN-γ exerted synergistic suppressive effects on the growth of KT-1 cells. Furthermore, IFN-α suppressed the expression of the bcr-ablfusion gene in KT-1 cells, and induced G1 cell-cycle arrest and apoptotic cell death. The KT-1 cell line should be a valuable tool for studying the molecular mechanism of the suppression of Ph1clone cells from CML by IFN.

Suppression of Cell Proliferation and the Expression of abcr-abl Fusion Gene and Apoptotic Cell Death in a New Human Chronic Myelogenous Leukemia Cell Line, KT-1, by Interferon-α

A new human leukemia cell line, KT-1, was established from a patient in the blastic crisis phase of chronic myelogenous leukemia (CML). This cell line had a positive reaction for intracytoplasmic myeloperoxidase and two Philadelphia chromosomes (Ph1) [t(9;22)(q34;q11)] and lacked normal copies of chromosomes 9 and 22. Molecular characterization of the breakpoint in the t(9;22)(q34;q11) showed that KT-1 had a bcr-2/abl-2 splice junction. When the KT-1 cells were cultured with interferon (IFN)-α or IFN-γ, the growth of the cells were dose-dependently suppressed. IFN-α and IFN-γ exerted synergistic suppressive effects on the growth of KT-1 cells. Furthermore, IFN-α suppressed the expression of the bcr-ablfusion gene in KT-1 cells, and induced G1 cell-cycle arrest and apoptotic cell death. The KT-1 cell line should be a valuable tool for studying the molecular mechanism of the suppression of Ph1clone cells from CML by IFN.

Nitric oxide and thiol redox regulation of Janus kinase activity

The activation of Janus kinases (JAKs) is crucial for propagation of the proliferative response initiated by many cytokines. The proliferation of various cell lines, particularly those of hematopoietic origin, is also modulated by mediators of oxidative stress such as nitric oxide and thiol redox reagents. Herein we demonstrate that nitric oxide and other thiol oxidants can inhibit the autokinase activity of rat JAK2 in vitro, presumably through oxidation of crucial dithiols to disulfides within JAK2. The reduced form of JAK2 is the most active form, and the oxidized JAK2 form is inactive. Nitric oxide pretreatment of quiescent Ba/F3 cells also inhibits the interleukin 3-triggered in vivo activation of JAK2, a phenomenon that correlates with inhibited proliferation. Furthermore, we observed that the autokinase activity of JAK3 responds in a similar fashion to thiol redox reagents in vitro and to nitric oxide donors in vivo. We suggest that the thiol redox regulation of JAKs may partially explain the generally immunosuppressive effects of nitric oxide and of other thiol oxidants.

Structural and mechanistic aspects of Janus kinases: how the two-faced god wields a double-edged sword

The Janus family of protein-tyrosine kinases has long been known to function in signal transduction pathways initiated by a host of cytokines. A brief overview of the role of Janus kinases (Jaks) in both cytokine and noncytokine signaling pathways highlights the broad physiologic importance of this kinase family. New insights into the structural and mechanistic regulatory aspects of Janus kinases are rapidly emerging. Recent mutational analyses allow the dissection of Jaks into three distinct structural domains governing receptor affiliation, autoregulation, and catalysis. A fourth domain determining substrate specificity is as yet poorly defined and is, therefore, discussed in the context of known substrates and inhibitors, a collection of molecules that have been expanded recently to include Stam and Jab. The proposed mechanism of the interconversion of Janus kinases from inactive to fully active enzymes involves three states of enzymatic activity. Additional layers of regulation can be independently superimposed on this multistate model, providing a simplified description of the behavior of Janus kinases under normal and pathologic circumstances.

Biomolecular interaction analysis of IFN gamma-induced signaling events in whole-cell lysates: prevalence of latent STAT1 in high-molecular weight complexes

The basic framework for the JAK/STAT pathway is well documented. Recruitment of latent cytoplasmic STAT transcription factors to tyrosine phosphorylated docking sites on cytokine receptors and their JAK-mediated phosphorylation instigates their translocation to the nucleus and their ability to bind DNA. The biochemical processes underlying recruitment and activation of this pathway have commonly been studied in reconstituted in vitro systems using previously defined recombinant signaling components. We have dissected the Interferon gamma (IFN gamma) signal transduction pathway in crude extracts from wild-type and STAT1-negative mutant cell lines by real-time BIAcore analysis, size-exclusion (SE) chromatography and immuno-detection. The data indicate that in detergent-free cell extracts: (1) the phospho-tyrosine (Y440P)-containing peptide motif of the IFN gamma-receptor alpha-chain interacts directly with STAT1, or STAT1 complexes, and no other protein; (2) non-activated STAT1 is present in a higher molecular weight complex(es) and, at least for IFN gamma-primed cells, is available for recruitment to the activated IFN gamma-receptor from only a subset of such complexes; (3) activated STAT1 is released from the receptor as a monomer.

In vivo class II transactivator expression in mice is induced by a non-interferon-gamma mechanism in response to local injury

BACKGROUND

Tissue injury induces MHC class II expression, which could be important in the recognition of that tissue as an allograft. The class II transcriptional activator (CIITA) is the major regulator of basal and induced MHC class II expression and is essential for antigen presentation. The role of CIITA in the induction of class II by tissue injury is unknown. In this study, we examined CIITA induction in the course of acute ischemic or toxic renal injury in mice, including the role of interferon (IFN)-gamma and of the transcription factor, interferon regulatory factor (IRF)-1.

METHODS

Kidneys were injured by ischemia or by gentamicin toxicity and were then studied for changes in gene expression using Northern blot, reverse transcriptase-polymerase chain reaction, radioimmunoassay, and tissue staining. We compared wild-type (WT) mice to IFN-gamma knockout (GKO) or IRF-1 knockout mice.

RESULTS

Ischemic injury induced CIITA and class II expression in the kidney, in WT and GKO mice. Gentamicin injury also induced both CIITA and class II expression, independent of IFN-gamma, in WT and GKO mice. After ischemic injury, the induction of class II protein levels and CIITA and class II mRNA levels were induced, to a lesser degree, in IRF-1 knockout mice.

CONCLUSIONS

These data indicate that CIITA is induced by tissue injury, and probably accounts for class II induction during tissue injury. CIITA induction by injury is largely IFN-gamma independent but requires IRF-1. The similarities of the pattern of CIITA and class II induction in ischemic and toxic injury suggest that this is a stereotyped response of injured tissue and not a consequence of a particular mechanism of injury.

Distinct tyrosine phosphorylation sites in JAK3 kinase domain positively and negatively regulate its enzymatic activity

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. Here, we investigated the role of tyrosine phosphorylation of JAK3 in regulating its kinase activity 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 individually or doubly. We found that JAK3 is autophosphorylated on multiple sites including Y980 and Y981. Compared with the activity of wild-type (WT) JAK3, mutant Y980F demonstrated markedly decreased kinase activity, and optimal phosphorylation of JAK3 on other sites was dependent on Y980 phosphorylation. The mutant Y980F also exhibited reduced phosphorylation of its substrates, gammac and STAT5A. In contrast, mutant Y981F had greatly increased kinase activity, whereas the double mutant, YY980/981FF, had 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 in 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.

Jaks, Stats and the immune system

Changes in gene expression are necessary for an adaptive response of cells to immunological stimuli and thus for their proper function in the context of the immune system. Regulatory inputs usually originate from cell surface receptors and in many cases affect the transcription rates of specific genes by modulating the activity of transcription factors. The Jak-Stat signalling paradigm has received large attention by molecular immunologists because it applies to nuclear signalling by all cytokine receptors. In its simplest form it requires only two protein components downstream of the receptor: Janus family protein tyrosine kinases (Jaks) which are usually receptor-associated, and signal transducer and activator of transcription (Stat) family transcription factors which carry the receptor-generated signal to the nucleus and stimulate gene expression. Here we give a brief overview of both recent progress and open questions concerning the Jak and Stat molecules, their regulation, and the biological implications of their activity.

Advances in the understanding of cytokine signal transduction: the role of Jaks and STATs in immunoregulation and the pathogenesis of immunodeficiency

Cytokines are of great importance in the growth and differentiation of hematopoietic and other cells. Moreover, they are also crucial in immunoregulation and in host defense. Although our understanding of the molecular basis of cytokine action is far from complete, recent advances have substantially improved our knowledge of cytokine-dependent signal transduction. The delineation of the structure of cytokine receptors and the signaling pathways they utilize has provided clues as to how the strikingly specific effects of cytokines are achieved. Additionally, the basis of some of the pleiotropic and redundant effects of cytokines has also become clear. The discovery of the Janus family of protein tyrosine kinases (Jaks) and the STATs (signal transducers and activators of transcription) has also provided key insights into the mechanism by which intracellular signals are transduced. The following paradigm has emerged: cytokines induce dimerization of receptor subunits that are constitutively associated with Jaks. This activates the Jaks, which then phosphorylate the receptors. The phosphorylated receptors are bound by SH2-containing proteins, one class of which is the STATs. Activated STATs, then, translocate to the nucleus to effect gene transcription. Though the Jaks do not explain much in terms of specificity in signaling, the function of the STATs does. The discovery of patients with autosomal recessive severe combined immunodeficiency due to mutations of a particular Jak, Jak3, and the phenotype of knockout mice lacking Jak3 and various STATs demonstrate the specific and critical roles of these molecules in the development and function of the immune system.

Stat1 serine phosphorylation occurs independently of tyrosine phosphorylation and requires an activated Jak2 kinase

Gamma interferon (IFN-gamma) induces both tyrosine and serine phosphorylation of Stat1. Stat1 serine phosphorylation is required for maximal transcriptional activity of Stat1. In this report, we present evidence that Stat1 tyrosine phosphorylation is not a prerequisite for Stat1 serine phosphorylation, although an active Jak2 kinase is required for both phosphorylation events. Stat1 serine phosphorylation occurs with a more delayed time course than tyrosine phosphorylation. The occurrence of serine phosphorylation without tyrosine phosphorylation suggests that serine phosphorylation takes place in the cytoplasm. Experiments performed with cells expressing either dominant-negative or constitutively active Ras protein indicated that the Ras-mitogen-activated protein kinase pathway is probably not involved in IFN-gamma-induced Stat1 serine phosphorylation. Finally, a kinase capable of correct Stat1 serine phosphorylation was detected in partially purified cytoplasmic extracts from both IFN-gamma-treated and untreated cells.

Tyrosine docking sites of the rat prolactin receptor required for association and activation of stat5

Prolactin (PRL) interacts with a single chain prolactin-specific receptor of the cytokine receptor superfamily. PRL triggers activation of Jak2 kinase which phosphorylates the PRL receptor itself and the mammary gland factor, Stat5, a member of the family of signal transducers and activators of transcription (Stat). Selection of the particular substrate (Stat 5), that is characterized by transcriptional responses to PRL, has been shown to be determined by specific tyrosine-based motifs common to many cytokine receptors. PRL-induced activation of Stat5 was abolished in 293 fibroblasts expressing PRL receptor mutants lacking all intracellular tyrosines. We have identified tyrosine phosphorylation sites of the PRL receptor (residues 580, 479, and 473) necessary for maximal Stat5 activation and subsequent Stat5-dependent gene transcription. Moreover, we have shown that none of the tyrosine residues of the PRL receptor are implicated in activation of Jak2. This study demonstrates that only specific tyrosines in the PRL receptor are phosphorylated and are in fact utilized differentially for Stat5-mediated transcriptional signaling.