Maximal activation of transcription by Stat1 and Stat3 requires both tyrosine and serine phosphorylation

Beta interferon and oncostatin M activate Raf-1 and mitogen-activated protein kinase through a JAK1-dependent pathway

Activation of early response genes by interferons (IFNs) and other cytokines requires tyrosine phosphorylation of a family of transcription factors termed signal transducers and activators of transcription (Stats). The Janus family of tyrosine kinases (Jak1, Jak2, Jak3, and Tyk2) is required for cytokine-induced tyrosine phosphorylation and dimerization of the Stat proteins. In order for IFNs to stimulate maximal expression of Stat1alpha-regulated genes, phosphorylation of a serine residue in the carboxy terminus by mitogen-activated protein kinase (MAPK) is also required. In HeLa cells, both IFN-beta and oncostatin M (OSM) stimulated MAPK and Raf-1 enzyme activity, in addition to Stat1 and Stat3 tyrosine phosphorylation. OSM stimulation of Raf-1 correlated with GTP loading of Ras, whereas IFN-beta activation of Raf-1 was Ras independent. IFN-beta- and OSM-induced Raf-1 activity could be coimmunoprecipitated with either Jak1 or Tyk2. Furthermore, HeLa cells lacking Jak1 displayed no activation of STAT1alpha, STAT3, and Raf-1 by IFN-beta or OSM and also demonstrated no increase in the relative level of GTP-bound p21ras in response to OSM. The requirement for Jak1 for IFN-beta- and OSM-induced activation of Raf-1 was also seen in Jak1-deficient U4A fibrosarcoma cells. Interestingly, basal MAPK, but not Raf-1, activity was constitutively enhanced in Jak1-deficient HeLa cells. Transient expression of Jak1 in both Jak-deficient HeLa cells and U4A cells reconstituted the ability of IFN-beta and OSM to activate Raf-1 and decreased the basal activity of MAPK, while expression of a kinase-inactive form of the protein showed no effect. Moreover, U4A cells selected for stable expression of Jak1, or COS cells transiently expressing Jak1 or Tyk2 but not Jak3, exhibited enhanced Raf-1 activity. Therefore, it appears that Jak1 is required for Raf-1 activation by both IFN-beta and OSM. These results provide evidence for a link between the Jaks and the Raf/MAPK signaling pathways.

A distinct function of STAT proteins in erythropoietin signal transduction

The Janus kinase (JAK)-signal transducers and activators of transcription (STAT) pathway is an important signaling pathway of interferons and cytokines. We examined the activation of STAT proteins induced by interleukin-3 (IL-3), granulocyte-macrophage colony-stimulating factor (GM-CSF), or erythropoietin (EPO) using the human leukemia cell line, UT-7, which requires these cytokines for growth. IL-3, GM-CSF, and EPO induced DNA-binding activity to the oligonucleotides corresponding to the sis-inducible elements (SIE) of c-fos, in addition to the beta-casein promoter (beta-CAP), SIE- and beta-CAP-binding proteins were identical to Stat1alpha and Stat3 complex and to Stat5 protein, respectively. This indicates that IL-3, GM-CSF, and EPO commonly activated Stat1alpha, Stat3, and Stat5 proteins in UT-7. However, EPO hardly activated Stat1alpha and Stat3 in UT-7/GM, which is a subline of UT-7 that grows slightly in response to EPO. Transfection studies revealed that UT-7/GM cells constitutively expressing Stat1alpha, but not Stat3, can grow as well in response to EPO as GM-CSF, suggesting that Stat1alpha is involved in the EPO-induced proliferation of UT-7. Thus, although Stat1alpha, Stat3, and Stat5 proteins are activated by GM-CSF, IL-3, and EPO, our data suggest that each STAT protein has a distinctive role in the actions of cytokines.

Box 3-independent signaling mechanisms are involved in leukemia inhibitory factor receptor alpha- and gp130-mediated stimulation of mitogen-activated protein kinase. Evidence for participation of multiple signaling pathways which converge at Ras

Chimeric receptors containing the entire or various cytoplasmic domains of either gp130 or leukemia inhibitory factor receptor alpha (LIFR) were used to identify signaling molecules and regions of these polypeptides required for the stimulation of mitogen-activated protein kinase (MAPK). Coexpression of dominant-negative Jak2 inhibited chimeric receptor-stimulated MAPK activity by approximately 70%, while expression of dominant-negative Ras completely blocked MAPK activation by either receptor polypeptide. Deletion analysis identified a 24-amino acid region of gp130 that was necessary for maximal stimulation of MAPK, and contained box 3 (positions 120-129) and a consensus tyrosine binding motif (Tyr-118) for the protein-tyrosine phosphatase, SHP2. Expression of receptors lacking this region or of chimeric gp130(Y118F) point mutants inhibited MAPK activity by approximately 55%, suggesting that Tyr-118, but not box 3, was required during activation of MAPK by gp130. Similarly, expression of chimeric LIFR constructs lacking box 3 maximally stimulated MAPK activity, while those lacking Tyr-115, a putative SHP2 binding site, inhibited stimulation of MAPK by this polypeptide. Our results demonstrate that gp130 and LIFR stimulate MAPK activity through box 3-independent mechanisms involving: (i) effects at Tyr-118 and Tyr-115, respectively, for maximal stimulation of MAPK activity and (ii) a Jak/Tyk-dependent pathway that, together with Tyr-118- or Tyr-115-generated signals, converges at the level of Ras during activation of MAPK by cytokine.

Novel pathway of insulin signaling involving Stat1alpha in Hep3B cells

STAT proteins are important transcription factors that regulate cell growth and differentiation. To elucidate the molecular mechanisms of insulin actions, we have studied how insulin activates STAT proteins in Hep3B cells. Insulin rapidly phosphorylated Stat1alpha at tyrosine residues and increased its specific binding activities to a GAS/ISRE consensus oligonucleotide. IL-4 also phosphorylated Stat1alpha and increased DNA binding activities to the same Stat1alpha responsive element. There was no increase in tyrosine phosphorylation of JAK family of kinases following insulin stimulation. In contrast, IL-4 stimulated tyrosine phosphorylation of JAK1, JAK2 and tyk2 in this cell line. These data indicate that insulin receptor signaling can activate the transcriptional regulatory function of STAT protein, and that insulin actions on Stat1alpha are mediated through signaling pathways independent of JAK family of kinases.

Interleukin-10 Inhibits Interferon-γ–Induced Intercellular Adhesion Molecule-1 Gene Transcription in Human Monocytes

Interleukin-10 (IL-10) is a potent monocyte regulatory cytokine that inhibits gene expression of proinflammatory mediators. In this study, we investigated the mechanism by which IL-10 downregulates expression of intercellular adhesion molecule-1 (ICAM-1) on the cell surface of normal human monocytes activated with interferon-γ (IFN-γ). IL-10 inhibition of IFN-γ–induced ICAM-1 expression was apparent as early as 3 hours and was blocked by an anti–IL-10 antibody but not by an isotype-matched control antibody. Northern blot analysis showed that IL-10 reduced the accumulation of ICAM-1 mRNA in IFN-γ–stimulated monocytes. IL-10 inhibition of ICAM-1 steady-state mRNA was detected at 3 hours and remained at 24 hours. Nuclear run-on transcription assays showed that IL-10 inhibited the rate of IFN-γ–induced transcription of the ICAM-1 gene, and mRNA stability studies showed that IL-10 did not alter the half-life of IFN-γ–induced ICAM-1 message. Thus, IL-10 inhibits IFN-γ–induced ICAM-1 expression in monocytes primarily at the level of gene transcription. Activation of IFN-γ–responsive genes requires tyrosine phosphorylation of the transcriptional factor STAT-1α (signal transducer and activator of transcription-1α). However, IL-10 did not affect IFN-γ–induced tyrosine phosphorylation of STAT-1α or alter STAT-1α binding to the IFN-γ response element (IRE) in the ICAM-1 promoter. Instead, IL-10 prevented IFN-γ–induced binding activity at the NF-κB site of the tumor necrosis factor α (TNF-α)–responsive NF-κB/C-EBP composite element in the ICAM-1 promoter. These data indicate that IL-10 inhibits IFN-γ–induced transcription of the ICAM-1 gene by a regulatory mechanism that may involve NF-κB.

Two discrete regions of interleukin-2 (IL2) receptor beta independently mediate IL2 activation of a PD98059/rapamycin/wortmannin-insensitive Stat5a/b serine kinase

Many cytokines, hormones, and growth factors activate Janus kinases to tyrosine phosphorylate select members of the Stat transcription factors. For full transcriptional activation, Stat1 and Stat3 also require phosphorylation of a conserved serine residue within a mitogen-activated protein kinase phosphorylation consensus site. On the other hand, two recently identified and highly homologous Stat5a and Stat5b proteins lack this putative mitogen-activated protein kinase phosphorylation site. The present study set out to establish whether Stat5a and Stat5b are under the control of an interleukin-2 (IL2)-activated Stat5 serine kinase. We now report that IL2 stimulated marked phosphorylation of serine and tyrosine residues of both Stat5a and Stat5b in human T lymphocytes and in several IL2-responsive lymphocytic cell lines. No Stat5a/b phosphothreonine was detected. Phosphoamino acid analysis also revealed that Stat5a/b phosphotyrosine levels were maximized within 1-5 min of IL2 stimulation, whereas serine phosphorylation kinetics were slower. Interestingly, IL2-induced serine phosphorylation of Stat5a differed quantitatively and temporally from that of Stat5b with Stat5a serine phosphorylation leveling off after 10 min and the more pronounced Stat5b response continuing to rise for at least 60 min of IL2 stimulation. Furthermore, we identified two discrete domains of IL2 receptor beta (IL2Rbeta) that could independently restore the ability of a truncated IL2Rbeta mutant to mediate Stat5a/b phosphorylation and DNA binding to the gamma-activated site of the beta-casein gene promoter. These observations demonstrated that there is no strict requirement for one particular IL2Rbeta region for Stat5 phosphorylation. Finally, we established that the IL2-activated Stat5a/b serine kinase is insensitive to several selective inhibitors of known IL2-stimulated kinases including MEK1/MEK2 (PD98059), mTOR (rapamycin), and phosphatidylinositol 3-kinase (wortmannin) as determined by phosphoamino acid and DNA binding analysis, thus suggesting that a yet-to-be-identified serine kinase mediates Stat5a/b activation.

The carboxyl-terminal region of STAT3 controls gene induction by the mouse haptoglobin promoter

Haptoglobin (HP) is one of the major acute phase plasma proteins in the mouse, and its synthesis is additively induced by interleukin (IL)-6 and glucocorticoids. STAT3 serves as the mediator of the IL-6 receptor signal and appears to contribute to the transcriptional induction of acute phase protein genes. The carboxyl-terminal region of STAT3, consisting of an acidic domain and containing a serine phosphorylation site, has been proposed to contribute to the induction process. To assess the role of STAT3 in the transcriptional control of the HP promoter, we applied two mutant forms of STAT3: one with a deletion of the carboxyl-terminal 55 amino acid residues, STAT3Delta55C, and the other with a substitution of serine 727 to alanine, STAT3SA. Like the wild-type STAT3, both mutant STAT3 forms are activated by the signal-transducing subunit of the IL-6 receptor, gp130, or by co-transfected IL-3 receptor. Ectopic expression and activation of wild-type STAT3 or STAT3SA in HepG2 hepatoma cells similarly enhance transcription through the IL-6-response element of the HP promoter. This enhancement is specific for STAT3 and cannot be reproduced by STAT1 or STAT5. In contrast, STAT3Delta55C inhibits IL-6-induced transcriptional activation. Interestingly, whereas receptor-activated STAT3 also enhances stimulation of the haptoglobin promoter by dexamethasone through the glucocorticoid receptor, activated STAT3Delta55C reduces the regulation below the level achieved by the glucocorticoid receptor alone. This transdominant action by STAT3Delta55C is dependent on a functional IL-6-responsive element. The data suggest that the carboxyl-terminal domain, but not its serine phosphorylation site of STAT3, is required for transcription as part of the hematopoietin receptor signaling as well as for cooperation with other transcription factors such as the glucocorticoid receptor.

Molecular role of TGF-beta, secreted from a new type of CD4+ suppressor T cell, NY4.2, in the prevention of autoimmune IDDM in NOD mice

A new type of CD4+ T cell clone (NY4.2) isolated from pancreatic islet-infiltrated lymphocytes of acutely diabetic non-obese diabetic (NOD) mice prevents the development of insulin-dependent diabetes mellitus (IDDM) in NOD mice, as well as the recurrence of autoimmune diabetes in syngeneic islet-transplanted NOD mice. It has been demonstrated that the cytokine TGF-beta, secreted from the cells of this clone, is the substance which prevents autoimmune IDDM. This investigation was initiated to determine the molecular role TGF-beta plays in the prevention of autoimmune IDDM by determining its effect on IL-2-induced signal transduction in Con A-activated NOD mouse splenocytes and HT-2 cells. First, we determined whether TGF-beta, secreted from NY4.2 T cells, inhibits IL-2-dependent T cell proliferation in HT-2 cells (IL-2-dependent T cell line) and NOD splenocytes. We found that TGF-beta suppresses IL-2-dependent T cell proliferation. Second, we determined whether TGF-beta inhibits the activation of Janus kinases (JAKs), as well as signal transducers and activators of transcription (STAT) proteins, involved in an IL-2-induced signalling pathway that normally leads to the proliferation of T cells. We found that TGF-beta inhibited tyrosine phosphorylation of JAK1, JAK3, STAT3 and STAT5 in Con A blasts from NOD splenocytes and HT-2 cells. Third, we examined whether TGF-beta inhibits the cooperation between STAT proteins and mitogen-activated protein kinase (MAPK), especially extracellular signal-regulated kinase 2 (ERK2). We found that TGF-beta inhibited the association of STAT3 and STAT5 with ERK2 in Con A blasts from NOD splenocytes and HT-2 cells. On the basis of these observations, we conclude that TGF-beta may interfere with signal transduction via inhibition of the IL-2-induced JAK/STAT pathway and inhibition of the association of STAT proteins with ERK2 in T cells from NOD splenocytes, resulting in the inhibition of IL-2-dependent T cell proliferation. TGF-beta-mediated suppression of T cell activation may be responsible for the prevention of effector T cell-mediated autoimmune IDDM in NOD mice by TGF-beta-producing CD4+ suppressor T cells.

Mitogenic stimulation of resting T cells causes rapid phosphorylation of the transcription factor LSF and increased DNA-binding activity

The mammalian transcription factor LSF (CP2/LBP-1c) binds cellular promoters modulated by cell growth signals. We demonstrate here that LSF-DNA-binding activity is strikingly regulated by induction of cell growth in human peripheral T lymphocytes. Within 15 min of mitogenic stimulation of these cells, the level of LSF-DNA-binding activity increased by a factor of five. The level of LSF protein in the nucleus remained constant throughout this interval. However, a rapid decrease in the electrophoretic mobility of LSF, attributable to phosphorylation, correlated with the increase in DNA-binding activity. pp44 (ERK1) phosphorylated LSF in vitro on the same residue that was phosphorylated in vivo, specifically at amino acid position 291, as indicated by mutant analysis. As direct verification of the causal relationship between phosphorylation and DNA-binding activity, treatment in vitro of LSF with phosphatase both increased the electrophoretic mobility of the protein and decreased LSF-DNA-binding activity. This modulation of LSF-DNA-binding activity as T cells progress from a resting to a replicating state reveals that LSF activity is regulated during cell growth and suggests that LSF regulates growth-responsive promoters.

Mapping of Stat3 serine phosphorylation to a single residue (727) and evidence that serine phosphorylation has no influence on DNA binding of Stat1 and Stat3

During their polypeptide ligand-induced activation Stats (signaltransducers andactivators oftranscription) 1 and 3 acquire, in addition to an obligatory tyrosine phosphorylation, phosphorylation on serine which boosts their transactivating potential [Wen, Z., Zhong, Z. and Darnell, J. E. Jr. (1995) Cell 82, 241-250]. By examining phosphopeptide maps of wild-type and mutant protein we show here that the Stat3 serine phosphorylation, like the Stat1 serine phosphorylation, occurs on a single residue, serine 727. Neither the DNA binding of Stat1 nor Stat3 is demonstrably affected by the presence or absence of the serine phosphorylation. Thus the earlier demonstration that transcription is enhanced by the presence of the serine 727 residue likely occurs after DNA binding. These findings do not agree with earlier claims of excess serine to tyrosine phosphorylation in activated Stats 1 and 3 or to claims of more stable DNA binding of serine phosphorylated Stat dimers.

Prolactin stimulates serine/tyrosine phosphorylation and formation of heterocomplexes of multiple Stat5 isoforms in Nb2 lymphocytes

Transcription factors of the Stat gene family are selectively activated by many hormones and cytokines. Stat5 originally was cloned as a prolactin-stimulated DNA-binding protein, but is also activated by non-lactogenic cytokines in many cell types. The recent identification of two distinct Stat5 genes, which encode a 94-kDa Stat5a and a 92-kDa Stat5b as well as several lower molecular weight isoforms, suggests additional complexity and combinatorial possibilities for transcriptional regulation. We now report a biochemical analysis of prolactin activation of Stat proteins in Nb2 lymphocytes, which was associated with: 1) rapid tyrosine phosphorylation of Stat5a, Stat5b, a COOH-terminally truncated 80-kDa Stat5 form, Stat1alpha, and Stat3; 2) rapid and selective formation of Stat5a/b heterodimers, without involvement of Stat1alpha or Stat3; 3) marked serine, but not threonine phosphorylation of Stat5a and Stat5b; and 4) the appearance of two qualitatively distinct Stat5 protein complexes, which discriminated between oligonucleotides corresponding to the prolactin response elements of the beta-casein and interferon regulatory factor-1 gene promoters. Collectively, our analyses showed that Stat5a and Stat5b respond similarly to prolactin receptor activation, but also suggested that the two genes have evolved unique properties that may contribute to the specificity of receptors that utilize Stat5 signaling proteins.

STAT3 as an adapter to couple phosphatidylinositol 3-kinase to the IFNAR1 chain of the type I interferon receptor

STAT (signal transducers and activators of transcription) proteins undergo cytokine-dependent phosphorylation on serine and tyrosine. STAT3, a transcription factor for acute phase response genes, was found to act as an adapter molecule in signal transduction from the type I interferon receptor. STAT3 bound to a conserved sequence in the cytoplasmic tail of the IFNAR1 chain of the receptor and underwent interferon-dependent tyrosine phosphorylation. The p85 regulatory subunit of phosphatidylinositol 3-kinase, which activates a series of serine kinases, bound to phosphorylated STAT3 and subsequently underwent tyrosine phosphorylation. Thus, STAT3 acts as an adapter to couple another signaling pathway to the interferon receptor.

Leptin induces mitogen-activated protein kinase-dependent proliferation of C3H10T1/2 cells

Leptin, secreted by adipocytes, regulates satiety and energy expenditure. Several forms of leptin receptors produced by alternative mRNA splicing are found in many tissues, including the hypothalamus, liver, lung, kidney, hematopoietic cells, and gonads, suggesting that leptin exerts effects in these tissues. In accordance with the distribution of leptin receptors, there is accumulating evidence that leptin plays various roles in reproduction, hematopoiesis, and the immune systems in addition to the regulation of food intake and energy expenditure. In the present study, we examined the in vitro effects of leptin on proliferation of a mouse embryonic cell line, C3H10T1/2, and its mechanism of action. Leptin caused a dose- and time-dependent increase in mitogen-activated protein kinase (MAPK) activity that was accompanied by an increase in C3H10T1/2 cell number. The MAPK kinase-1-specific inhibitor PD98059 completely blocked the increases in both MAPK activity and cell proliferation caused by leptin. These findings indicate that leptin stimulates the proliferation of C3H10T1/2 cells via the MAPK cascade.

Growth hormone and phorbol esters require specific protein kinase C isoforms to activate mitogen-activated protein kinases in 3T3-F442A cells

Previous studies have shown that the activation of p44 and p42 mitogen-activated protein (MAP) kinases (ERK1 and ERK2) by growth hormone (GH) and phorbol esters, but not by epidermal growth factor, in 3T3-F442A preadipocytes is dependent on protein kinase C (PKC). In the present study two approaches have been taken to determine the PKC isoform dependence of MAP kinase activation in these cells. By immunoblotting with specific antibodies, the cells were found to express PKC-alpha, -gamma,-delta, -epsilon and -zeta. Treatment of cells with 500 nM PMA for 3 h led to the complete depletion of PKC-delta and the partial depletion of PKC-alpha but did not significantly affect the expression of the other PKC isoforms. In parallel, such treatment severely attenuated the ability of GH to activate MAP kinase. The degree of this attenuation was not increased by more prolonged PMA pretreatment, indicating that PKC-delta and perhaps PKC-alpha are important for MAP kinase activation by GH. These experiments further revealed that additional PKC isoforms were required for the full activation of MAP kinases by acute treatment with PMA. A second approach involved the use of anti-sense oligodeoxynucleotides (ODNs) to deplete the individual PKC isoforms selectively. Each of the ODNs used effectively depleted the relevant isoform to undetectable levels and did not affect the expression of the other PKC isoforms. Pretreatment of cells with PKC-delta anti-sense ODN, but not with anti-sense ODN to the other phorbol ester-sensitive isoforms, severely attenuated the activation of MAP kinases by GH. PKC-delta anti-sense ODN also blocked (by approx. 50%) the activation of MAP kinases by PMA. Furthermore a combination of PKC-delta and -epsilon anti-sense ODNs completely blocked the effect of PMA on MAP kinases. Collectively, these results indicate that the novel PKC-delta and -epsilon isoforms can couple to the MAP kinase pathway in 3T3-F442A cells but that the activation of MAP kinases by GH specifically involves PKC-delta.

Activation of Stat1 and subsequent transcription of inducible nitric oxide synthase gene in C6 glioma cells is independent of interferon-gamma-induced MAPK activation that is mediated by p21ras

Rat C6 glioma cells have been used to characterize molecular events involved in the regulation of inducible nitric oxide synthase (iNOS) gene expression stimulated by interferon-gamma (IFN-gamma) plus lipopolysaccharide (LPS). IFNs induce a signaling event which involves activation of Stat1 transcription factor. Previous studies have shown that IFNs also induce extracellular signal-regulated kinase/mitogen-activated protein kinase (ERK/MAPK) activation. However, the mechanisms by which IFNs stimulate MAPK activation remain elusive. Here we show that in C6 glioma cells, transiently expressing the dominant-negative form of c-Ha-Ras (Asn-17) abrogated IFN-gamma-induced ERK1 and ERK2 activation. Furthermore, PD98059, a specific MEK1 inhibitor, also blocked this activation. These results indicate that p21ras and MEK1 are required for IFN-gamma-induced ERK1 and ERK2 activation. Recent studies have reported that MAPK is responsible for serine phosphorylation of Stat1 which is required for Stat1's DNA binding and maximal transcriptional activity. Thus, we examined the role of the Ras-MAPK pathway in Stat1 activation and subsequent iNOS induction in C6 glioma cells. Further experiments showed that neither Asn-17 Ras expression nor concentrations of PD98059, which completely abrogated IFN-gamma-induced ERK1 and ERK2 activation, affected Stat1 DNA binding activity or iNOS induction, indicating that the Ras-MAPK pathway does not appear to be involved in the activation of Stat1 and subsequent iNOS induction in C6 glioma cells.

Receptors That Induce Erythroid Differentiation of Ba/F3 Cells: Structural Requirements and Effect on STAT5 Binding

Ectopic expression of the erythropoietin receptor (EpoR) in the interleukin-3 (IL-3)–dependent cell line Ba/F3 results in growth and partial erythroid differentiation in Epo. In contrast, introduction and activation of the interleukin-5 receptor (IL-5R) or of the granulocyte-macrophage colony-stimulating factor receptor (GM-CSFR) results in proliferation only. As this effect is specific to the EpoR, the role of its extracellular or cytoplasmic domain in differentiation was tested after construction of two chimeric receptors. One receptor contained the extracellular domain of EpoR fused to the endodomain of IL-3R β-chain (E/β), while the other contained the EpoR cytoplasmic region fused to the extracellular domain of GM-CSFR α-chain (GMER). Surprisingly, both receptors induced differentiation ruling out a strict specificity of the extracellular or cytoplasmic region of EpoR in this process. Instead the ability to signal differentiation correlated with structural features shared by the EpoR, GMER, and E/β receptors. Dimerization of all three receptors results in the pairing of two signal transducing chains in the cytoplasm, in contrast to the mitogenic receptors IL-3R, IL-5R, GM-CSFR, which assemble as αβ heterodimers. Two new chimeric receptors that fulfilled the structural requirement exemplified by EpoR, but lacked any part of EpoR, were designed to consolidate this model. They consisted of the ectodomains of the GMR-α and IL-5Rα, respectively, fused to the endodomain of IL-3R β-chain. Both receptors were as effective as EpoR in signaling differentiation in response to their cognate ligand. Another property of receptors fulfilling these structural requirements is that they cause a marked delay in signal transducers and activators of transcription 5 (STAT5) activation on ligand stimulation. Taken together our studies show that structural assembly of receptors dictates their potential to signal erythroid differentiation in Ba/F3 cells, that differentiation can take place in the absence of Epo and that a delay in STAT5 activation is highly predictive of this process.

Angiotensin II stimulates rapid serine phosphorylation of transcription factor Stat3

In rat neonatal cardiac fibroblasts and CHO-K1 cells expressing angiotensin type 1 receptors, angiotensin II (AII) rapidly caused a time dependent reduction in the SDS-polyacrylamide gel electrophoretic mobility of Stat3 (Signal Transducer and Activator of Transcription). This was concentration dependent and detected at a low/physiological concentration of AII (1 nM), with initial effect observed as early as 2 min; and maximal at 5 min. The rapid stimulation of Stat3 mobility retardation by AII, paralleled the rapid activation of MAP kinases (mitogen-activated protein kinases), and both were sensitive to the MAP kinase kinase 1 inhibitor, PD98059. Immunoprecipitation of Stat3 from [32P] labeled cells demonstrated a 4-fold increase in Stat3 phosphorylation in response to AII, and phosphoamino acid analysis indicated that phosphorylation occurred on serine residues. Angiotensin II-induced rapid phosphorylation of Stat3 was also sensitive to the MAP kinase kinase 1 inhibitor, PD98059. Treatment of immunoprecipitated Stat3 from AII-treated cells with protein phosphatase- PP-2A, reversed the AII-induced retardation of Stat3 mobility. These results demonstrate that AII rapidly induces Stat3 serine phosphorylation through a MAP kinase kinase 1 dependent pathway. Rapid stimulation of Stat3 serine phosphorylation by AII may have implications in the modulation of its transcriptional activity and gene expression.

Regulation by cAMP of STAT1 activation in hepatic stellate cells

Previously we reported that dibutyryl cAMP and phosphodiesterase inhibitor methylxanthines block rat stellate cell proliferation. To analyze the underlying mechanism, modulation by these agents of platelet-derived growth factor (PDGF)/BB-stimulating signal pathway was studied. Without reducing STAT1 protein level, these agents were found to attenuate STAT1 activation in stellate cells stimulated with PDGF/BB as revealed by an electrophoretic mobility shift assay. Inhibitory effect started 12 h after exposure of the cells to these agents at concentrations of more than 100 microM. These agents had no effects on DNA binding activity of STAT1 that had already been activated. Treatment with these agents failed to affect the function of PDGF receptors except for partial attenuation of phospholipase C activation under PDGF/BB stimulation. The present results indicate that inhibition of STAT1 activation may be one of factors involved in the cAMP-dependent stellate cell growth arrest.

MNK1, a new MAP kinase-activated protein kinase, isolated by a novel expression screening method for identifying protein kinase substrates

We have developed a novel expression screening method for identifying protein kinase substrates. In this method, a lambda phage cDNA expression library is screened by in situ, solid-phase phosphorylation using purified protein kinase and [gamma-32P]ATP. Screening a HeLa cDNA library with ERK1 MAP kinase yielded cDNAs of previously characterized ERK substrates, c-Myc and p90RSK, demonstrating the utility of this method for identifying physiological protein kinase substrates. A novel clone isolated in this screen, designated MNK1, encodes a protein-serine/threonine kinase, which is most similar to MAP kinase-activated protein kinase 2 (MAPKAP-K2), 3pK/MAPKAP-K3 and p90RSK. Bacterially expressed MNK1 was phosphorylated and activated in vitro by ERK1 and p38 MAP kinases but not by JNK/SAPK. Further, MNK1 was activated upon stimulation of HeLa cells with 12-O-tetradecanoylphorbol-13-acetate, fetal calf serum, anisomycin, UV irradiation, tumor necrosis factor-alpha, interleukin-1beta, or osmotic shock, and the activation by these stimuli was differentially inhibited by the MEK inhibitor PD098059 or the p38 MAP kinase inhibitor SB202190. Together, these results indicate that MNK1 is a novel class of protein kinase that is activated through both the ERK and p38 MAP kinase signaling pathways.

Integration of Jak-Stat and AP-1 signaling pathways at the vasoactive intestinal peptide cytokine response element regulates ciliary neurotrophic factor-dependent transcription

Ciliary neurotrophic factor (CNTF)-dependent induction of expression of the neuropeptide vasoactive intestinal peptide (VIP) gene is mediated by a 180-base pair cytokine response element (CyRE) in the VIP promoter. To elucidate the molecular mechanisms mediating the transcriptional activation by CNTF, intracellular signaling to the CyRE has been studied in a neuroblastoma cell line. It has been shown previously that CNTF induces Stat proteins to bind to a site within the CyRE. CNTF also induces a second protein to bind to a C/EBP-like site within the CyRE. In this report, we show that this inducible CyRE binding protein is composed of the AP-1 proteins c-Fos, JunB, and JunD. These proteins bind to a non-canonical AP-1 site located near the previously characterized C/EBP site. The serine/threonine kinase inhibitor H7 prevents CNTF-dependent induction of AP-1 binding and CyRE-mediated transcription, suggesting that an H7-sensitive kinase is important to mediating CNTF effects on VIP transcription. The integration at the VIP CyRE of the Jak-Stat and AP-1 signaling pathways with other pre-existing proteins provides a cellular mechanism for cell- and cytokine-specific signaling.

Phosphorylation events modulate the ability of interferon consensus sequence binding protein to interact with interferon regulatory factors and to bind DNA

Two families of transcription factors mediate interferon (IFN) signaling. The first family, signal transducers and activators of transcription (STATs), is activated within minutes of IFN treatment. Specific phosphorylation events lead to their translocation to the nucleus, formation of transcriptional complexes, and the induction of the second family of transcription factors termed interferon regulatory factors (IRFs). Interferon consensus sequence binding protein (ICSBP) is a member of IRF family that is expressed only in cells of the immune system and acts as a transcriptional repressor. ICSBP binds DNA through the association with other transcription factors such as IRF-1 or IRF-2. In this communication, the domain that is involved in protein-protein interactions was mapped to the carboxyl terminus of ICSBP. This domain is also important for mediating ICSBP-repressing activity. In vitro studies demonstrated that direct binding of ICSBP to DNA is prevented by tyrosine (Tyr) phosphorylation. Yet, Tyr-phosphorylated ICSBP can bind target DNA only through the association with IRF-2 and IRF-1. This type of phosphorylation is essential for the formation of heterocomplexes. Tyr-phosphorylated ICSBP and IRF-2 are detected in expressing cells constitutively, and Tyr-phosphorylated IRF-1 is induced by IFN-gamma. These results strongly suggest that like the STATs, the IRFs are also modulated by Tyr phosphorylation that affects their biological activities.

Binding sequence of STAT4: STAT4 complex recognizes the IFN-gamma activation site (GAS)-like sequence (T/A)TTCC(C/G)GGAA(T/A)

Studies of transcriptional activation by interferons and various cytokines have led to the identification of a family of proteins that serve as signal transducers and activators of transcription (STAT). STAT4 is phosphorylated following interleukin (IL)-12 stimulation and is required for IL-12 signal transduction. By immunoprecipitation and PCR amplification, a specific consensus sequence for DNA binding of the STAT4 complex was determined. The binding sequence of the STAT4 complex, (T/A)TTCC(C/G)GGAA(T/A), proved to be palindromic and similar to the IFN-gamma activated site (GAS)-like sequence. The first (T/A) and last (T/A) sites of the consensus sequence were critical for the binding affinity of the STAT4 complex.

Ras-independent transformation by v-Src

Signaling by a variety of receptor and nonreceptor tyrosine kinases is mediated by Ras, a membrane-associated GTPase. Expression of v-Src, a transforming nonreceptor tyrosine kinase, results in Ras activation, and inhibition of Ras function in NIH 3T3 cells suppresses transformation by v-Src, indicating that in these cells Ras-dependent signaling pathways are required for v-Src to exert its biological effects. However, we show here that Ras was not activated in Rat-2 fibroblasts transformed by wild-type v-Src, or in chicken embryo fibroblasts transformed by SRX5, a v-Src mutant with a linker insertion at the major site of autophosphorylation. Expression of a dominant-negative mutant of Ras completely inhibited the ability of v-Src to activate the mitogen-activated protein kinase ERK2, which is downstream of Ras. However, dominant-negative Ras did not suppress transformation by v-Src as judged by a variety of criteria. Thus, v-Src can transform at least some cell types in the absence of Ras activation or Ras-stimulated ERK2 activity, and in these cells activation of Ras-independent signaling pathways must therefore be sufficient for transformation.

Interferon-gamma induced increases in intracellular cathepsin B activity in THP-1 cells are dependent on RNA transcription

Interferon-gamma (IFN-gamma) treatment of human macrophages induces increased intracellular levels of cathepsin B (CB), a lysosomal cysteine proteinase which is implicated in inflammatory tissue injury. To determine the mechanism of the increase, we studied the macrophage-like cell line, THP-1. Dose and time dependent increases in intracellular CB were seen when cells primed with phorbol ester (PMA) were cultured with IFN-gamma. To determine whether protein synthesis was required for the increase, PMA primed cells were cultured in the presence of IFN-gamma and cycloheximide: The expected increase was inhibited. To determine whether RNA synthesis was required for the IFN-gamma induced increases, PMA primed cells were cultured in the presence of IFN-gamma and actinomycin D. Again the expected increases were not seen. Direct measurement of CB mRNA levels showed increases in cells not treated with inhibitors. These results suggest that the IFN-gamma induced increases in THP-1 cell CB are dependent on RNA and protein synthesis.

Differential regulation of cytokine-induced major histocompatibility complex class II expression and nitric oxide release in rat microglia and astrocytes by effectors of tyrosine kinase, protein kinase C, and cAMP

Two glial cell populations of the CNS, astrocytes and microglia, were examined for expression of two immunologically important molecules, MHC class II and nitric oxide (NO), following treatment with cytokines. IFN-gamma induced both molecules in microglia at substantially higher levels than astrocytes. The addition of TNF-alpha to IFN-gamma elevated class II expression and NO in both cells. Genistein, an inhibitor of tyrosine kinases, and calphostin, an inhibitor of protein kinase C, diminished cytokine induction of class II MHC and NO in both glial populations. Forskolin was most effective in inhibiting class II MHC expression, but had little inhibitory effect on NO production. These results indicate microglia are more effective than astrocytes in producing cell-associated and secreted immune mediators in response to IFN-gamma and or TNF-alpha and multiple parallel, but distinct, signaling events are required for cytokine induced class II MHC or NO production.

Regulation of signal transducer and activator of transcription (STAT) 5b activation by the temporal pattern of growth hormone stimulation

Plasma GH profiles, intermittent in adult male and continuous in adult female rats, respectively, activate unique patterns of gene transcription in male and female rat liver. Pulsatile, but not continuous, GH exposure activates liver STAT5 (signal transducer and activator of transcription-5) by tyrosine phosphorylation, leading to nuclear translocation, and is proposed to play a key role in GH pulse-regulated male-specific liver gene expression. The mechanisms underlying the GH pattern dependence of STAT5 activation are presently investigated using a rat hepatocyte-derived cell line. Rat GH stimulated tyrosine phosphorylation followed by serine or threonine phosphorylation, leading to activation of the DNA-binding activity of STAT5b, the major STAT5 form present in these cells. Maximal STAT5b activation required a full 20 min at a receptor-saturating GH concentration of 50 ng/ml, suggesting that hormone binding leading to receptor dimerization is a relatively slow process. Repeat cycles of GH pulsation led to repeat cycles of STAT5b activation followed by deactivation, similar to rat liver in vivo. Full responsiveness to succeeding GH pulses required a minimum GH off-time of > or = 2.5 h, but was independent of new protein synthesis. Continuous GH exposure led to down-regulation of activated STAT5b, consistent with the desensitization of this GH pulse-activated pathway observed in female rat liver. The rapid deactivation of STAT5b after termination of a GH pulse involved phosphotyrosine dephosphorylation as a key first step and could be blocked by pervanadate, a phosphotyrosine phosphatase inhibitor. Unexpectedly, serine/threonine kinase inhibitors also inhibited STAT5b deactivation. These studies establish that STAT5b is responsive to the temporal pattern of GH stimulation and demonstrate a role for both a tyrosine phosphatase and a serine/threonine kinase in resetting this JAK/STAT signaling apparatus so that it may respond to subsequent rounds of GH pulse activation.

THE IFNγ RECEPTOR:A Paradigm for Cytokine Receptor Signaling

▪ Abstract  During the last several years, the mechanism of IFNγ-dependent signal transduction has been the focus of intense investigation. This research has recently culminated in the elucidation of a comprehensive molecular understanding of the events that underlie IFNγ-induced cellular responses. The structure and function of the IFNγ receptor have been defined. The mechanism of IFNγ signal transduction has been largely elucidated, and the physiologic relevance of this process validated. Most recently, the molecular events that link receptor ligation to signal transduction have been established. Together these insights have produced a model of IFNγ signaling that is nearly complete and that serves as a paradigm for signaling by other members of the cytokine receptor superfamily.

The state of the STATs: recent developments in the study of signal transduction to the nucleus

The STATs (signal transducers and activators of transcription) are latent cytoplasmic proteins that, upon activation by cell surface bound polypeptide ligands, move to the nucleus to direct transcription. A variety of protein-protein interactions that affect the function of STATs has been recently recognized. It has become clear that the STATs are functional mosaics, or mixtures of signal transduction and transcription modules.

Prolactin signal transduction to milk protein genes: carboxy-terminal part of the prolactin receptor and its tyrosine phosphorylation are not obligatory for JAK2 and STAT5 activation

In this study, we have developed several Chinese Hamster ovary (CHO) cell clones stably expressing various deletion mutant forms of the rabbit prolactin receptor (rbPRL-R) to better define the domains of the receptor involved in JAK2 kinase interaction, STAT5 activation, and to assess the role of tyrosine phosphorylation of the PRL-R in signal transduction. We observed that the box 1 region of the receptor was critical for productive interaction with JAK2 and its tyrosine phosphorylation after PRL stimulation. However, this region appeared to require the presence of additional cytoplasmic domain region(s), such as box 2, to exert its complete effect. In addition, we found that a mutant form lacking the 141 C-terminal residues lost the capacity to be tyrosine phosphorylated in response to PRL but remained able to activate JAK2 kinase and STAT5 transcription factor, indicating that it contained the minimal sequence required for STAT5 activation. The absence of tyrosine phosphorylation of this C-terminal rbPRL-R mutant upon PRL stimulation indicated that the phosphorylation of the PRL-R normally occured in the last 141 animo acids (aa) containing three tyrosines and was not absolutely necessary for induction of these early events in PRL signal transduction. Transfectant cell lines expressing wild type (WT) PRL-R and this C-terminal mutant form were able to induce CAT activity upon PRL stimulation when transiently transfected with the ovine-beta-lactoglobulin promoter, containing STAT5 recognition sites, fused to the CAT reporter gene. The comparison between transcriptional activity of these two receptor forms leads to the conclusion that the C-terminal region of the rbPRL-R, containing the physiological sites for tyrosine phosphorylation, is probably responsible for an amplification of the PRL signal to milk protein genes.

Signal transducer and activator of transcription-3 (STAT3) is constitutively activated in normal, self-renewing B-1 cells but only inducibly expressed in conventional B lymphocytes

Cytokine and growth factor receptor engagement leads to the rapid phosphorylation and activation of latent, cytosolic signal transducers and activators of transcription (STAT) proteins, which then translocate to the nucleus where they regulate transcriptional events from specific promoter sequences. STAT3 expression in particular has been associated with Abl, Src, and HTLV-1 transformation of normal cells. B-1 lymphocytes are self-renewing, CD5+ B cells that display a propensity for malignant transformation and are the normal counterpart to human chronic lymphocytic leukemias. Further, B-1 cells are characterized by aberrant intracellular signaling, including hyperresponsiveness to phorbol ester PKC agonists. Here we demonstrate that B-1 lymphocytes constitutively express nuclear activated STAT3, which is not expressed by unmanipulated conventional (B-2) lymphocytes. In contrast, STAT3 activation is induced in B-2 cells after antigen receptor engagement in a delayed fashion (after 3 h). Induction of STAT3 is inhibited by both the serine/threonine protein kinase inhibitor H-7 and the immunosuppressive drug rapamycin and requires de novo protein synthesis, demonstrating novel coupling between sIg and STAT proteins that differs from the classical paradigm for STAT induction by cytokine receptors. The inability of prolonged stimulation of conventional B-2 cells with anti-Ig, a treatment sufficient to induce CD5 expression, to result in sustained STAT3 activation suggests that STAT3 is a specific nuclear marker for B-1 cells. Thus, STAT3 may play a role in B cell antigen-specific signaling responses, and its constitutive activation is associated with a normal cell population exhibiting intrinsic proliferative behavior.

Physical association between STAT1 and the interferon-inducible protein kinase PKR and implications for interferon and double-stranded RNA signaling pathways

The interferon-inducible double-stranded RNA protein kinase PKR controls protein synthesis through the phosphorylation of eukaryotic translation initiation factor (eIF)-2. In addition to its demonstrated role in translational control, several reports have suggested a transcriptional role for PKR. Here we report that PKR is involved in IFN- and dsRNA-signaling pathways by modulating the function of the signal transducer and activator of transcription STAT1. We also show that PKR associates with STAT1 in mouse and human cells. The association is not a kinase-substrate interaction since STAT1 phosphorylation is not modified by PKR in vitro or in vivo. In addition, the formation of the PKR-STAT1 complex is not dependent upon the enzymatic activity of PKR but does require the dsRNA-binding domain of PKR. Moreover, there is a concomitant decrease in PKR-STAT1 interaction and increase in STAT1 DNA binding in response to IFNs or dsRNA. These findings suggest that PKR plays an important role in IFN and dsRNA-signaling pathways by modulating the transcriptional function of STAT1.

GAS elements: a few nucleotides with a major impact on cytokine-induced gene expression

Gamma interferon activation site (GAS) elements are short stretches of DNA, originally defined as a requirement for the rapid transcriptional induction of genes in response to interferon-gamma (IFN-gamma). The protein complex binding to GAS sequences in IFN-gamma-treated cells, the gamma interferon activation factor (GAF), is a dimer of Stat1, the prototype of a family of cytokine-responsive transcription factors, the signal transducers and activators of transcription. To date, seven different Stats are known (excluding alternatively spliced or processed forms), six of which recognize the same small palindromic consensus sequence TTCN2-4 GAA that defines a GAS element. Because one or several Stats take part in nuclear signaling in response to most cytokines or growth factors, the GAS sequence has changed from being viewed as a specific site for IFN-activated GAF to becoming the general nuclear end of the Jak-Stat signaling pathways. This review focuses on the identification and definition of GAS elements, their interaction with Stat transcription factors, and their contribution to the specificity of cytokine-induced gene expression.

Nuclear integration of JAK/STAT and Ras/AP-1 signaling by CBP and p300

We report that interferon gamma (IFN-gamma) inhibits transcription of the macrophage scavenger receptor gene by antagonizing the Ras-dependent activities of AP-1 and cooperating ets domain transcription factors, apparently as a result of competition between AP-1/ets factors and activated STAT1 for limiting amounts of CBP and p300. Consistent with this model, STAT1 alpha interacts directly with CBP in cells, and microinjection of anti-CBP and anti-p300 antibodies blocks transcriptional responses to IFN-gamma. Cells lacking STAT1 fail to inhibit AP-1/ets activity, and overexpression of CBP both potentiates IFN-gamma-dependent transcription and relieves AP-1/ets repression. Thus, CBP and p300 integrate both positive and negative effects of IFN-gamma on gene expression by serving as essential coactivators of STAT1 alpha, modulating gene-specific responses to simultaneous activation of two or more signal transduction pathways.

The hsp90-binding antibiotic geldanamycin decreases Raf levels and epidermal growth factor signaling without disrupting formation of signaling complexes or reducing the specific enzymatic activity of Raf kinase

We have expressed the mitogenic signaling proteins Src, Ras, Raf-1, Mek (MAP kinase kinase), and Erk (MAP kinase) in baculovirus-infected Sf9 insect cells in order to study a potential role for the chaperone hsp90 in formation of multiprotein complexes. One such complex obtained by immunoadsorption with anti-Ras antibody of cytosol prepared from cells simultaneously expressing Ras, Raf, Mek, and Erk contained Ras, Raf, and Erk. To detect directly the protein-protein interactions involved in forming multiprotein complexes, we combined cytosols from single infections in vitro in all possible combinations of protein pairs. We detected complexes between Ras.Raf, Ras.Src, Raf.Mek, and Raf.Src, but no complex containing Erk was obtained by mixing cytosols. Thus, cellular factors appear to be required for assembly of the Erk-containing multiprotein complex. One cellular factor thought to be involved in signaling protein complex formation is the chaperone hsp90, and we show that Src, Raf, and Mek are each complexed with insect hsp90. Treatment of Sf9 cells with geldanamycin, a benzoquinone ansamycin that binds to hsp90 and disrupts its function, did not decrease coadsorption of either Raf or Erk with Ras, although it did decrease the level of cytosolic Raf. To study geldanamycin action, we treated rat 3Y1 fibroblasts expressing v-Raf and showed that the antibiotic blocked assembly of Raf.hsp90 complexes at an intermediate stage of assembly where Raf is still bound to the p60 and hsp70 components of the assembly mechanism. As in Sf9 cells, Raf levels decline with geldanamycin treatment of 3Y1 cells. To determine if geldanamycin affects mitogenic response, we treated HeLa cells with epidermal growth factor (EGF) and showed that geldanamycin treatment decreased EGF signaling and decreased the level of Raf protein without affecting the EGF-mediated increase in Raf kinase activity. We conclude that hsp90 is not required for forming complexes between the mitogenic signaling proteins or for Raf kinase activity and that EGF signaling is decreased indirectly by geldanamycin because the antibiotic increases degradation of Raf and perhaps other components of the signaling pathway.

The signal transducer gp130 is shared by interleukin-6 family of haematopoietic and neurotrophic cytokines

Receptors for many of the cytokines functioning in the haematopoietic system belong to the class I cytokine receptor family. In most cases these receptors share common signal transducing receptor components in the same family, which explains the functional redundancy of haematopoietic cytokines. Interleukin-6 and related cytokines, interleukin-11, leukaemia inhibitory factor, oncostatin M, ciliary neurotrophic factor and cardiotrophin-1, are all pleiotrophic, from the haematopoietic to the nervous system, and exhibit overlapping biological activities. Receptors for these cytokines fall into the class I cytokine receptor family. Functional receptor complexes for the interleukin-6 family of cytokines share a membrane glycoprotein 130 (gp130) as a critical component for signal transduction. In these receptor complexes, gp130 and ligand-specific chains possess no intrinsic tyrosine kinase domain but are associated with cytoplasmic tyrosine kinases. Ligand stimulation triggers homo- or heterodimerization of gp130, leading to activation of the associated cytoplasmic tyrosine kinases and subsequent modification of transcription factors. This paper reviews the recent progress in the study of gp130 and the background information from biomedical and biochemical viewpoints.

Shared promoter elements between a viral superantigen and the major histocompatibility complex class II-associated invariant chain

Superantigens have the ability to stimulate subsets of T lymphocytes bearing particular T-cell receptor Vbeta chains. The best-known viral superantigen is Mls, a product of the murine mammary tumor virus (MMTV) sag gene. The MMTV superantigen is not displayed by the virus itself; however, after infection of B lymphocytes, the superantigen is expressed. The resulting immune stimulation is essential for viral transmission. We have analyzed the transcriptional elements which control Mls-1 expression. Here we present evidence that a region at the 3' end of Mtv-7 env, Penv2, controls B-cell-specific expression of sag. Penv2 has elements homologous with promoters of immunoglobulin H chain, the invariant chain, and major histocompatibility complex class II, suggesting a coordinate regulation of expression of these various B-cell-specific genes and indicating a possible eukaryotic origin of MMTV sag. We have determined that both an IgH heptamer element and a Y box are essential for Penv2 promoter activity and that tandem octamer motifs in the U3 region of the 3' MMTV long terminal repeat function as enhancers. We propose that Penv2 controls constitutive Mls expression in B lymphocytes.

Constitutive activation of JAK1 in Src-transformed cells

We have previously found that the signal transducer and activator of transcription (Stat) 3 is constitutively activated in cells stably transformed by the v-Src oncoprotein. While activation of Stat proteins has also been observed following epidermal growth factor or platelet-derived growth factor stimulation, Stat3 activation is more commonly associated with signaling through cytokine receptors and activation of the Janus family tyrosine kinases JAK1 or JAK2. We therefore investigated whether JAK1 or JAK2 were activated in Src-transformed cells. In three v-Src-transformed fibroblast cell lines (NIH3T3, Balb/c, and 3Y1), JAK1 displayed increased tyrosyl phosphorylation compared to non-transformed cells. The level of tyrosyl phosphorylation of JAK1 was significantly greater in NIH3T3 cells transformed by expression of v-Src or high levels of a constitutively active mutant of c-Src (Y527F) than in cells overexpressing the less transforming normal c-Src. Enzymatic activity of JAK1 was assessed using autophosphorylation assays. In anti-JAK1 immunoprecipitates from v-Src-transformed NIH3T3 cells, a protein with the same migration as JAK1 showed substantially increased levels of 32P incorporation compared to immunoprecipitates from non-transformed cells. Similar results were obtained using anti-JAK2 immunoprecipitates; however, the level of JAK2 tyrosyl phosphorylation and 32P incorporation in anti-JAK2 immunoprecipitates were markedly lower than in anti-JAK1 immunoprecipitates. We conclude that JAK1, and possibly JAK2, are constitutively activated in Src-transformed cells, raising the possibility that Janus family kinases contribute to the constitutive activation of Stat3 previously observed in these cells and/or other properties of Src-transformed cells.

Deficient cytokine signaling in mouse embryo fibroblasts with a targeted deletion in the PKR gene: role of IRF-1 and NF-kappaB

The interferon (IFN)-induced double-stranded RNA (dsRNA)-activated Ser/Thr protein kinase (PKR) plays a role in the antiviral and antiproliferative effects of IFN. PKR phosphorylates initiation factor eIF2alpha, thereby inhibiting protein synthesis, and also activates the transcription factor, nuclear factor-kappaB (NF-kappaB), by phosphorylating the inhibitor of NF-kappaB, IkappaB. Mice devoid of functional PKR (Pkr(o/o)) derived by targeted gene disruption exhibit a diminished response to IFN-gamma and poly(rI:rC) (pIC). In embryo fibroblasts derived from Pkr(o/o) mice, interferon regulatory factor 1 (IRF-1) or guanylate binding protein (Gbp) promoter-reporter constructs were unresponsive to IFN-gamma or pIC but response could be restored by co-transfection with PKR. The lack of responsiveness could be attributed to a diminished activation of IRF-1 and/or NF-kappaB in response to IFN-gamma or pIC. Thus, PKR acts as a signal transducer for IFN-stimulated genes dependent on the transcription factors IRF-1 and NF-kappaB.

The mitogen-activated protein kinase and JAK-STAT signaling pathways are required for an oncostatin M-responsive element-mediated activation of matrix metalloproteinase 1 gene expression

Both astrocytes in the central nervous system and fibroblasts in somatic tissues are not only the major sources of extracellular matrix components but also of matrix metalloproteinases (MMPs), a family of enzymes directly involved in extracellular matrix breakdown. We have analyzed the regulation of the expression of MMPs and TIMPs (tissue inhibitors of metalloproteinases) in human primary astrocytes stimulated with oncostatin M (OSM) and other extracellular mediators in comparison with normal human dermal fibroblasts. It was found that OSM induced/enhanced transcription of MMP-1 (interstitial collagenase) and MMP-3 (stromelysin 1) in astrocytes, and MMP-1, MMP-9 (gelatinase B), and TIMP-1 in fibroblasts. Analysis of the signal transduction leading to activation of the MMP-1 gene revealed the presence of an OSM-responsive element (OMRE) encompassing the AP-1 binding site and the signal transducer and activator of transcription (STAT) binding element, which mediate activation by OSM. OMRE is also present in the TIMP-1 gene promoter and, although there are some differences in these two motifs, both appear to be targets for the simultaneous action of OSM-induced nuclear effectors. The induced enhancement of transcription by synergistically acting AP-1 and STAT binding elements in response to OSM is Raf-dependent. Cross-talk between the mitogen-activated protein kinase and JAK-STAT pathways is required to achieve maximal induction of the OMRE-driven transcription by OSM.

Interferon gamma-dependent induction of human intercellular adhesion molecule-1 gene expression involves activation of a distinct STAT protein complex

In response to interferon gamma (IFNgamma), intercellular adhesion molecule-1 (ICAM-1) is expressed on human keratinocytes, a cell type that is critically involved in cutaneous inflammation. An ICAM-1 5' regulatory region palindromic response element, pIgammaRE, has been shown to confer IFNgamma-dependent transcription enhancement. By electrophoretic mobility shift assays (EMSA), pIgammaRE forms a distinct complex with proteins from IFNgamma-treated human keratinocytes, termed gamma response factor (GRF). Binding of GRF is tyrosine phosphorylation-dependent, and mutations of pIgammaRE that disrupt the palindromic sequence or alter its spatial relationship abrogate GRF binding. Supershift EMSAs using antibodies to characterized STAT proteins suggest that GRF contains a Stat1alpha-like protein; however, non-ICAM-1 IFNgamma-responsive elements (REs) known to bind Stat1alpha homodimers fail to compete for GRF binding in EMSA, and pIgammaRE does not cross-compete with these REs that complex with homodimeric stat1alpha. The pIgammaRE x GRF complex also displays a distinctly different electrophoretic mobility compared to that of IFNgammaREs complexed to homodimeric Stat1alpha. These findings indicate that a distinct complex containing a Stat1alpha-like protein mediates IFNgamma-induced ICAM-1 gene transcription and identifies a subset of IFNgamma-responsive genes that appear to be regulated by this complex.

Prolactin mediated intracellular signaling in mammary epithelial cells

Prolactin binds to a member of the cytokine receptor superfamily. The cytoplasmic domain of the prolactin receptor (PrlR) displays no enzymatic activity yet prolactin treatment leads to the induction of protein tyrosine phosphorylation. PrlR is associated with JAK2, a protein tyrosine kinase whose activity is stimulated following receptor dimerization. JAK2 subsequently phosphorylates PrlR and other cellular proteins which are recruited to the activated receptor complex. Among the JAK2 substrates is the transcription factor Stat5 whose phosphorylation mediates the transcriptional activation of beta-casein gene expression. In this review we discuss the prolactin induced signaling pathways which mediate differentiation of the mammary gland.

The IFN gamma receptor: a paradigm for cytokine receptor signaling

During the last several years, the mechanism of IFN gamma-dependent signal transduction has been the focus of intense investigation. This research has recently culminated in the elucidation of a comprehensive molecular understanding of the events that underlie IFN gamma-induced cellular responses. The structure and function of the IFN gamma receptor have been defined. The mechanism of IFN gamma signal transduction has been largely elucidated, and the physiologic relevance of this process validated. Most recently, the molecular events that link receptor ligation to signal transduction have been established. Together these insights have produced a model of IFN gamma signaling that is nearly complete and that serves as a paradigm for signaling by other members of the cytokine receptor superfamily.

STAT1 pathway is involved in activation of caprine arthritis-encephalitis virus long terminal repeat in monocytes

The caprine arthritis-encephalitis virus (CAEV) long terminal repeat (LTR) is activated by gamma interferon (IFN-gamma) in promonocytic cells. We have previously shown that a 70-bp element is necessary and sufficient for the response of the CAEV LTR to this cytokine. At the 5' end, this 70-bp IFN-gamma response element contains sequence similarity to the gamma activated site (GAS). Here we demonstrate that the putative GAS element in the CAEV LTR binds specifically to a cellular factor induced by IFN-gamma in promonocytic cells. Substitution mutations in this consensus sequence eliminate binding of the inducible factor. The GAS element from the 70-bp motif is sufficient to confer responsiveness to IFN-gamma using a heterologous minimal promoter. Consistent with the binding data, the same mutations in the GAS element eliminate responsiveness to IFN-gamma in the context of both a functional CAEV LTR and a heterologous promoter. The cellular factor that binds to the GAS element is present from 5 min to 14 h after stimulation with IFN-gamma. Binding of the nuclear factor to the GAS element in the CAEV LTR is inhibited by antibody directed against STAT1 (p91/84). Thus, the GAS sequence in the CAEV LTR is essential for the response to IFN-gamma and a STAT1-like factor binds to this site. The STAT-1 signaling pathway provides at least one mechanism for activation of the CAEV LTR by IFN-gamma in monocytes. These data are the first demonstration of a role for a STAT family member in the regulation of a viral promoter.

Gp130 and the interleukin-6 family of cytokines

Receptors for most interleukins and cytokines that regulate immune and hematopoietic systems belong to the class I cytokine receptor family. These molecules form multichain receptor complexes in order to exhibit high-affinity binding to, and mediate biological functions of, their respective cytokines. In most cases, these functional receptor complexes share common signal transducing receptor components that are also in the class I cytokine receptor family, i.e. gp130, common beta, and common gamma molecules. Interleukin-6 and related cytokines, interleukin-11, leukemia inhibitory factor, oncostatin M, ciliary neurotrophic factor, and cardiotrophin-1 are all pleiotropic and exhibit overlapping biological functions. Functional receptor complexes for this interleukin-6 family of cytokines share gp130 as a component critical for signal transduction. Unlike cytokines sharing common beta and common gamma chains that mainly function in hematopoietic and lymphoid cell systems, the interleukin-6 family of cytokines function extensively outside these systems as well, e.g. from the cardiovascular to the nervous system, owing to ubiquitously expressed gp130. Stimulation of cells with the interleukin-6 family of cytokines triggers homo- or hetero-dimerization of gp130. Although gp130 and its dimer partners possess no intrinsic tyrosine kinase domain, the dimerization of gp130 leads to activation of associated cytoplasmic tyrosine kinases and subsequent modification of transcription factors. This paper reviews recent progress in the study of the interleukin-6 family of cytokines and gp130.

Requirement for JAK2 in erythropoietin-induced signalling pathways

Erythropoietin (EPO) exerts its activities by the induction of multiple signalling pathways through interaction with the erythropoietin receptor (EPOR). Previous studies have suggested that the Ras/MAP kinase as well as the JAK/STAT signalling cascades play significant roles in the induction of EPO-responsive genes. Here we show that, in HCD-57 erythroleukemic cells, both pathways are activated by EPO in a dose-dependent manner with similar sensitivities and kinetics. The activation of signalling molecules is closely related to the proliferative status of the cells. Using an antisense strategy, we were able to show that the downregulation of the JAK2 protein level in HCD-57 cells results in a distinct reduction of the ability to induce not only STAT5 DNA-binding, but also MAP kinase activity. Our results thus provide evidence for a significant contribution of the cytosolic tyrosine kinase JAK2 to the EPO-induced activation of the Ras/MAP kinase cascade.

Two contact regions between Stat1 and CBP/p300 in interferon gamma signaling

Interferon gamma (IFN-gamma) induces rapid tyrosine phosphorylation of the latent cytoplasmic transcription factor, Stat1, which then forms homodimers, translocates to the nucleus and participates in IFN-gamma-induced transcription. However, little is known of the interactions between Stat1 and the general transcription machinery during transcriptional activation. We show here that Stat1 can directly interact with the CREB-binding protein (CBP)/p300 family of transcriptional coactivators. Specifically, two interaction regions were identified: the amino-terminal region of Stat1 interacts with the CREB-binding domain of CBP/p300 and the carboxyl-terminal region of Stat1 interacts with the domain of CBP/p300 that binds adenovirus E1A protein. Transfection experiments suggest a role for these interactions in IFN-gamma-induced transcription. Because CBP/p300-binding is required for the adenovirus E1A protein to regulate transcription of many genes during viral replication and cellular transformation, it is possible that the anti-viral effect of IFN-gamma is based at least in part on direct competition by nuclear Stat1 with E1A for CBP/p300 binding.

Lactogenic hormone activation of Stat5 and transcription of the beta-casein gene in mammary epithelial cells is independent of p42 ERK2 mitogen-activated protein kinase activity

HC11 mammary epithelial cells have been used to characterize molecular events involved in the regulation of milk protein gene expression. Treatment of HC11 cells with the lactogenic hormones prolactin, insulin, and glucocorticoids results in transcription of the beta-casein gene. Prolactin induces a signaling event which involves tyrosine phosphorylation of the mammary gland factor, Stat5, a member of the family of signal transducers and activators of transcription (Stat). Here we show that HC11 cells express two Stat5 proteins, Stat5a and Stat5b. Phosphopeptide and phosphoamino acid analysis of Stat5a and Stat5b immunoprecipitated from phosphate-labeled HC11 cells revealed that both proteins were constitutively phosphorylated on serine. Lactogenic hormone treatment resulted in the appearance of a tyrosine-phosphorylated peptide in both Stat5 proteins. Consistent with this observation, a Western blot analysis of Stat5a and Stat5b showed that lactogenic hormones induced a rapid, transient increase in phosphotyrosine which paralleled the binding of Stat5 to its cognate recognition sequence in the beta-casein gene promoter. Lactogenic hormone treatment of the HC11 cells also led to a rapid activation of the mitogen-activated protein (MAP) kinase pathway. We examined the role of this pathway in beta-casein transcription using a specific MAP kinase kinase inhibitor, PD98059. Concentrations of PD98059 which completely abrogated lactogen-induced MAP kinase activation did not affect the phosphorylation state of Stat5, its DNA binding activity, or transcriptional activation of a beta-casein reporter construct. This indicates that the MAP kinase pathway does not contribute to lactogenic hormone induction of the beta-casein gene.

Elf-1 and Stat5 bind to a critical element in a new enhancer of the human interleukin-2 receptor alpha gene

The interleukin 2 receptor alpha-chain (IL-2R alpha) gene is a key regulator of lymphocyte proliferation. IL-2R alpha is rapidly and potently induced in T cells in response to mitogenic stimuli. Interleukin 2 (IL-2) stimulates IL-2R alpha. transcription, thereby amplifying expression of its own high-affinity receptor. IL-2R alpha transcription is at least in part controlled by two positive regulatory regions, PRRI and PRRII. PRRI is an inducible proximal enhancer, located between nucleotides -276 and -244, which contains NF-kappaB and SRE/CArG motifs. PRRII is a T-cell-specific enhancer, located between nucleotides -137 and -64, which binds the T-cell-specific Ets protein Elf-1 and HMG-I(Y) proteins. However, none of these proximal regions account for the induction of IL-2R alpha transcription by IL-2. To find new regulatory regions of the IL-2R alpha gene, 8.5 kb of the 5' end noncoding sequence of the IL-2R alpha gene have been sequenced. We identified an 86-nucleotide fragment that is 90% identical to the recently characterized murine IL-2-responsive element (mIL-2rE). This putative human IL-2rE, designated PRRIII, confers IL-2 responsiveness on a heterologous promoter. PRRIII contains a Stat protein binding site that overlaps with an EBS motif (GASd/EBSd). These are essential for IL-2 inducibility of PRRIII/CAT reporter constructs. IL-2 induced the binding of Stat5a and b proteins to the human GASd element. To confirm the physiological relevance of these findings, we carried out in vivo footprinting experiments which showed that stimulation of IL-2R alpha expression correlated with occupancy of the GASd element. Our data demonstrate a major role of the GASd/EBSd element in IL-2R alpha regulation and suggest that the T-cell-specific Elf-1 factor can serve as a transcriptional repressor.