A single STAT recruitment module in a chimeric cytokine receptor complex is sufficient for STAT activation

Dynamics of receptor and protein transducer homodimerisation

Background

Signalling pathways are complex systems in which not only simple monomeric molecules interact, but also more complex structures that include constitutive or induced protein assemblies. In particular, the hetero-and homo-dimerisation of proteins is a commonly encountered motif in signalling pathways. Several authors have suggested in recent times that dimerisation relates to a series of physical and biological outcomes used by the cell in the regulation of signal transduction.

Results

In this paper we investigate the role of homodimerisation in receptor-protein transducer interactions. Towards this end, mathematical modelling is used to analyse the features of such kind of interactions and to predict the behaviour of the system under different experimental conditions. A kinetic model in which the interaction between homodimers provokes a dual mechanism of activation (single and double protein transducer activation at the same time) is proposed. In addition, we analyse under which conditions the use of a power-law representation for the system is useful. Furthermore, we investigate the dynamical consequences of this dual mechanism and compare the performance of the system in different simulated experimental conditions.

Conclusion

The analysis of our mathematical model suggests that in receptor-protein interacting systems with dual mechanism there may be a shift between double and single activation in a way that intense double protein transducer activation could initiate and dominate the signal in the short term (getting a fast intense signal), while single protein activation could control the system in the medium and long term (when input signal is weaker and decreases slowly). Our investigation suggests that homodimerisation and oligomerisation are mechanisms used to enhance and regulate the dynamic properties of the initial steps in signalling pathways.

Deviant effects in molecular reaction pathways

In biological networks, any manifestations of behaviors substantially 'deviant' from the predictions of continuous-deterministic classical chemical kinetics (CCK) are typically ascribed to systems with complex dynamics and/or a small number of molecules. Here we show that in certain cases such restrictions are not obligatory for CCK to be largely incorrect. By systematically identifying properties that may cause significant divergences between CCK and the more accurate discrete-stochastic chemical master equation (CME) system descriptions, we comprehensively characterize potential CCK failure patterns in biological settings, including consequences of the assertion that CCK is closer to the 'mode' rather than the 'average' of stochastic reaction dynamics, as generally perceived. We demonstrate that mechanisms underlying such nonclassical effects can be very simple, are common in cellular networks and result in often unintuitive system behaviors. This highlights the importance of deviant effects in biotechnologically or biomedically relevant applications, and suggests some approaches to diagnosing them in situ.

Three Dileucine-like Motifs within the Interbox1/2 Region of the Human Oncostatin M Receptor Prevent Efficient Surface Expression in the Absence of an Associated Janus Kinase

The oncostatin M receptor (OSMR) is part of receptor complexes for oncostatin M and interleukin-31. Signaling events are triggered by Jaks (Janus kinases) that constitutively bind to membrane-proximal receptor regions. Besides their established role in signaling, Jaks are involved in the regulation of the surface expression of several cytokine receptors. Here, we analyzed the structural requirements within the human OSMR that underlie its limited surface expression in the absence of associated Jaks. We identified three dileucine-like motifs within the Jak-binding region of the OSMR that control receptor surface and overall expression. A receptor mutant in which all three motifs were mutated to alanine displayed markedly increased surface expression. Although the surface half-life of this mutant was increased compared with that of the wild-type receptor, no difference in the internalization rate was detectable, implying that these receptors differ in their post-endocytic fate. The protein stability of the wild-type receptor was markedly lower than that of mutant receptors, but could be strongly increased in the presence of the lysosomal inhibitor chloroquine. Our data are consistent with the dileucine motifs being involved in destabilization of receptors devoid of associated Jaks as part of a quality control ensuring signaling competence of OSMRs.

The Jak1 SH2 Domain Does Not Fulfill a Classical SH2 Function in Jak/STATSignaling but Plays a Structural Role for Receptor Interaction andUp-regulation of Receptor SurfaceExpression

The presence of a Src homology 2 (SH2) domain sequence similarity in the sequence of Janus kinases (Jaks) has been discussed since the first descriptions of these enzymes. We performed an in depth study to determine the function of the Jak1 SH2 domain. We investigated the functionality of the Jak1 SH2 domain by stably reconstituting Jak1-defective human fibrosarcoma cells U4C with endogenous amounts of Jak1 in which the crucial arginine residue Arg466 within the SH2 domain has been replaced by lysine. This mutant still binds to the receptor subunits gp130 and OSMR. Moreover, the SH2 R466K mutation does not affect the subcellular distribution of Jak1 as assessed by cell fractionation and confocal microscopy of cells expressing endogenous levels of non-tagged or a yellow fluorescent protein (YFP)-tagged Jak1-R466K, respectively. Likewise, the signaling capacity of Jak1 was not affected by this point mutation. However, we found that the SH2 domain is structurally important for cytokine receptor binding and surface expression of the OSMR.

Determinants governing the potency of STAT3 activation via the individual STAT3-recruiting motifs of gp130

In recent years, the elucidation of the structures of many signalling molecules has allowed new insights into the molecular mechanisms that govern signal transduction events. In the field of cytokine signalling, the solved structures of cytokine/receptor complexes and of key components involved in signal transduction such as STAT factors or the tyrosine phosphatase SHP2 have broadened our understanding of the molecular basis of the signalling events and provided key information for the rational design of therapeutic approaches to modulate or block cytokine signal transduction. Unfortunately, no structural data on the intracellular parts of cytokine receptors are available. The exact molecular mechanism underlying one of the first steps in signal transduction, namely the recruitment of signalling components to the cytoplasmic parts of cytokine receptors, remains elusive. Here we investigated possible mechanisms underlying the different potency of the STAT3-activating motifs of gp130 after IL-6 stimulation. Our data indicate that the extent of STAT3 activation by the different receptor motifs is not influenced by structural features such as contacts between the two gp130 chains. In addition, the proximity of the negatively regulating motif around tyrosine Y759 to the different STAT3-recruiting motifs does not seem to be responsible for their differential capacity to activate STAT3. However, the potency of a specific motif to activate STAT3 directly reflects the affinity for the binding of STAT3 to this motif.

The tyrosine 974 within the LIF-R-chain of the gp130/LIF-R heteromeric receptor complex mediates negative regulation of LIF signalling

Signalling of interleukin (IL)-6 and interleukin-11 through gp130 homodimeric receptor complexes has been analysed with respect to initiation and termination of signalling in great detail. Gp130 contains a crucial motif around tyrosine Y759, which mediates negative regulation through the feedback inhibitor SOCS3 and the protein tyrosine phosphatase SHP2. Signalling of leukaemia inhibitory factor (LIF), ciliary neurotrophic factor (CNTF), cardiotrophin-1 (CT-1), CT-1-like factor (CLC) or oncostatin M (OSM) through gp130/LIF-R is believed to be similar due to the presence of the common signal transducer gp130 within the receptor complexes utilized, but the difference in the composition of gp130/gp130-homodimers and gp130/LIF-R-heterodimers is likely to be reflected in different signalling. Here, we analysed the contribution of the LIF-R within the gp130/LIF-R complex to negative regulation mediated by SHP2 and SOCS3. We show that SHP2 contributes to the negative regulation of signalling through gp130/LIF-R complexes. The inhibitory tyrosine motifs within the cytoplasmic parts of gp130 and the LIF-R act independently. Whereas SHP2 and SOCS3 bind directly to the inhibitory motif of gp130, only SHP2 was found to bind to the corresponding inhibitory sequence of the LIF-R. This observation was further corroborated by experiments indicating that mainly gp130 contributes to the inhibition of signalling by SOCS3.

Identification of the cytoplasmic domains of CXCR4 involved in Jak2 and STAT3 phosphorylation

The chemokine SDF-1alpha transduces G(i)-dependent and -independent signals through CXCR4. Activation of Jak2/STAT3, a G(i)-independent signaling pathway, which plays a major role in survival signals, is known to be activated after SDF-1alpha binding to CXCR4 but the domains of CXCR4 involved in this signaling remain unexplored. Using human embryonic kidney HEK-293 cells stably expressing wild-type or mutated forms of CXCR4, we demonstrated that STAT3 phosphorylation requires the N-terminal part of the third intracellular loop (ICL3) and the tyrosine 157 present at the end of the second intracellular loop (ICL2) of CXCR4. In contrast, neither the conserved Tyr(135) in the DRY motif at the N terminus of ICL2 nor the Tyr(65) and Tyr(76) in the first intracellular loop (ICL1) are involved in this activation. ICL3, which does not contain any tyrosine residues, is needed to activate Jak2. These results demonstrate that two separate domains of CXCR4 are involved in Jak2/STAT3 signaling. The N-terminal part of ICL3 is needed to activate Jak2 after SDF-1alpha binding to CXCR4, leading to phosphorylation of only one cytoplasmic Tyr, present at the C terminus of ICL2, which triggers STAT3 activation. This work has profound implications for the understanding of CXCR4-transduced signaling.

Normal expression of IFN-gammaR in four patients with uncommon mycobacterial infection phenotypes

Several primary immunodeficiency diseases affecting the interleukin 12/interferon gamma (IFN-gamma) pathway have been identified, most of them characterized by recurrent and protracted infections produced by intracellular microorganisms, particularly by several species of mycobacteria. In the present study we analyzed the expression of IFN-gamma receptor (IFN-gammaR) and signal transducer and activator of transcription 1 (STAT-1) in 4 children with Mycobacterium tuberculosis infection of uncommon clinical presentation. These molecules were evaluated by flow cytometry and Western blotting in B cells transformed with Epstein-Barr virus and mutations were scanned by single-strand conformational polymorphisms and DNA sequencing. The expression of IFN-gammaR1 was normal in all 4 patients. The genetic analysis of IFN-gammaR1 and IFN-gammaR2 coding sequences did not reveal any mutation. The expression of the STAT-1 molecule was similar in patients and healthy controls; however, when the phosphorylation of this transcription factor in response to IFN-gamma activation was evaluated by Western blot, a significant lower signal was evident in one patient. These data indicate that there are no alterations in the expression or function of the IFN-gammaR chains in these patients. However, the low level of STAT-1 phosphorylation found in one of these patients might be explained by a defect in one of the molecules involved in the signal transduction pathway after IFN-gamma interacts with its receptor. In the other three patients the inability to eliminate the mycobacteria may be due to a defect in another effector mechanism of the mononuclear phagocytes.

Characterization of the Signaling Capacities of the Novel gp130-like Cytokine Receptor

The gp130-like receptor (GPL) is a recently cloned member of the family of type I cytokine receptors. The name reflects its close relationship to gp130, the common receptor subunit of the interleukin (IL)-6-type cytokines. Indeed, the recently proposed ligand for GPL, IL-31, is closely related to the IL-6-type cytokines oncostatin M, leukemia inhibitory factor, and cardiotrophin-1. The second signal transducing receptor for IL-31 seems to be the oncostatin M receptor beta (OSMRbeta). The present study characterizes in depth the molecular mechanisms underlying GPL-mediated signal transduction. GPL is a strong activator of STAT3 and STAT5, whereas STAT1 is only marginally tyrosine-phosphorylated. We identify tyrosine residues 652 and 721 in the cytoplasmic region of the longest isoform of GPL (GPL(745)) as the major STAT5- and STAT3-activating sites, respectively. Additionally, we demonstrate Jak1 binding to GPL and its activation in heteromeric complexes with the OSMRbeta but also in a homomeric receptor complex. Most interesting, unlike OSMRbeta and gp130, GPL is insufficient to mediate ERK1/2 phosphorylation. We propose that this is due to a lack of recruitment of the tyrosine phosphatase SHP-2 or the adaptor protein Shc to the cytoplasmic domain of GPL.

Bacteria Displaying Interleukin-4 Mutants Stimulate Mammalian Cells and Reflect the Biological Activities of Variant Soluble Cytokines

We describe a novel procedure that allows the rapid determination of cytokine activity on cells that express their cognate receptor. The four-helix bundle cytokine interleukin-4 (IL-4) was inducibly expressed as a fusion with the E. coli outer-membrane protein intimin, such that IL-4 was presented on the surfaces of the bacteria. Expression and accessibility of the cytokine on the cell exteriors were monitored by Western blotting and fluorescence microscopy, making use of two epitopes flanking the IL-4 component of the fusion protein. To demonstrate the biological activity of the immobilized cytokine, a Ba/F3-derived cell line stably transfected with both the bipartite human IL-4 receptor and an IL-4-specific luciferase reporter gene construct was employed. Bacterial cells displaying interleukin-4 elicited a specific, dose-dependent response in the reporter cells. Two variants of IL-4 with previously characterized (partial) antagonistic properties were also expressed as membrane-bound fusion proteins and were tested for their activity in the immobilized state. In comparison with bacteria displaying wild-type IL-4, E. coli clones presenting variants IL-4 Y124G and Y124D showed diminished or abolished activity, respectively, on murine reporter cells. The relative signaling potencies of the immobilized IL-4 variants thus closely mirror the agonistic properties of the corresponding soluble cytokines. This approach should be generally applicable for the mutational analysis of numerous signal mediators that trigger cellular responses through dimerization of transmembrane receptors.

Real Time Analysis of STAT3 Nucleocytoplasmic Shuttling

The transcription factor STAT3 is most important for the signal transduction of interleukin-6 and related cytokines. Upon stimulation cytoplasmic STAT3 is phosphorylated at tyrosine 705, translocates into the nucleus, and induces target genes. Notably, STAT proteins are also detectable in the nuclei of unstimulated cells. In this report we introduce a new method for the real time analysis of STAT3 nucleocytoplasmic shuttling in living cells which is based on the recently established fluorescence localization after photobleaching (FLAP) approach. STAT3 was C-terminally fused with the cyan (CFP) and yellow (YFP) variants of the green fluorescent protein. In the resulting STAT3-CFP-YFP (STAT3-CY) fusion protein the YFP can be selectively bleached using the 514-nm laser of a confocal microscope. This setting allows studies on the dynamics of STAT3 nucleocytoplasmic transport by monitoring the subcellular distribution of fluorescently labeled and selectively bleached STAT3-CY. By this means we demonstrate that STAT3-CY shuttles continuously between the cytosol and the nucleus in unstimulated cells. This constitutive shuttling does not depend on the phosphorylation of tyrosine 705 because a STAT3(Y705F)-CY mutant shuttles to the same extent as STAT3-CY. Experiments with deletion mutants reveal that the N-terminal moiety of STAT3 is essential for shuttling. Further studies suggest that a decrease in STAT3 nuclear export contributes to the nuclear accumulation of STAT3 in response to cytokine stimulation. The new approach presented in this study is generally applicable to any protein of interest for analyzing nucleocytoplasmic transport mechanisms in real time.

Preassociation of nonactivated STAT3 molecules demonstrated in living cells using bioluminescence resonance energy transfer: a new model of STAT activation?

Signal transducers and activators of transcription (STATs) are crucial molecules in cytokine signaling. In the conventional model of STAT activation, STAT molecules are recruited from a latent pool of cytoplasmic monomers to the activated cytokine receptor. After binding to the receptor, they get tyrosine-phosphorylated, dissociate from the receptor, and translocate to the nucleus as activation-induced dimers. Recently, several publications questioned this model of STAT activation and showed the existence of preassociated STAT molecules before activation. We were able to demonstrate the existence of these preassociated STAT3 molecules in living mammalian cells using bioluminescence resonance energy transfer. Our results support the new hypothesis that STAT molecules exist in the cytoplasm as dimers or multimers and point to an activation-induced change in STAT3 conformation. Therefore, we propose a new model of STAT activation and discuss a hypothetical structure of "cytoplasmic" STAT dimers as opposed to the known "activation-induced" dimer.

STAT3 is enriched in nuclear bodies

Signal transducer and activator of transcription 3 (STAT3) is a transcription factor that is involved in a variety of biological functions. It is essential for the signal transduction of interleukin-6 (IL-6) and related cytokines. In response to IL-6 stimulation STAT3 becomes phosphorylated and translocates into the nucleus where it binds to enhancer sequences of target genes. We found that activated STAT3 is enriched in dot-like structures within the nucleus, which we termed STAT3 nuclear bodies. To examine the dynamics of STAT3 nuclear body formation, a fusion protein of STAT3 and yellow fluorescent protein (YFP) was constructed. Studies in living cells have shown that the appearance of STAT3 nuclear bodies is transient, correlating with the timecourse of tyrosine-phosphorylation of STAT3. Furthermore, we show by fluorescence recovery after photobleaching (FRAP) analysis that STAT3 within nuclear bodies consists of a highly mobile and an immobile fraction. Colocalization studies provided evidence that these bodies are accompanied with CREB binding protein (CBP) and acetylated histone H4, which are markers for transcriptionally active chromatin. Moreover, STAT3 nuclear bodies in HepG2 cells are not colocalized with promyelocytic leukemia oncoprotein (PML)-containing bodies; neither is a sumoylation of activated STAT3 detectable. Taken together, our data suggest that STAT3 nuclear bodies are either directly involved in active gene transcription or they serve as reservoirs of activated STAT3.

Protein kinase C delta associates with the interleukin-6 receptor subunit glycoprotein (gp) 130 via Stat3 and enhances Stat3-gp130 interaction

The transcriptional regulation of Stat proteins is controlled through their C-terminal domains, which harbor both a tyrosine phosphorylation site, required for dimerization and subsequent nuclear translocation, and a serine phosphorylation site, required for maximum transcriptional activity. Previously, we reported that protein kinase Cdelta (PKCdelta) phosphorylates and interacts with Stat3 in an interleukin (IL)-6-dependent manner. In this study, we further characterized this interaction, and investigated the potential role of such an interaction. We show here that the catalytic domain of PKCdelta interacts with the Src homology 2 domain and part of the adjacent C-terminal transactivation domain of Stat3. This interaction, which does not seem to involve a classical phosphotyrosine SH2-mediated binding, however, significantly enhances the interaction of Stat3 and the IL-6 receptor subunit glycoprotein (gp) 130, which is the initial step for Stat3 activation by IL-6. Expression of a dominant negative PKCdelta or depletion of the endogenous PKCdelta by phorbol 12-myristate 3-acetate treatment abrogates the association of Stat3 with gp130. At the same time, PKCdelta is recruited to gp130 via association with Stat3, which may facilitate its phosphorylation on the gp130 receptor. Finally, we identified Thr-890, a putative PKC phosphorylation site on gp130, to be critical for the effect of PKCdelta. Our data indicate that PKCdelta plays important regulatory roles in IL-6 signaling.

Orientational constraints of the gp130 intracellular juxtamembrane domain for signaling

The glycoprotein 130 (gp130) is the common signal transducing receptor chain of the interleukin-6 family of cytokines. Here we investigated the requirements for transfer of the information given by ligand binding to the cytoplasmic domain of gp130. It is demonstrated that the box 1/2 region has to be located membrane-proximally in order to bind and activate Janus kinases. To test the possible requirement of an alpha-helical orientation, we inserted 1-4 alanine residues into this juxtamembrane intracellular region. The insertion of one alanine results in a strongly reduced activation of STAT1 and STAT3, whereas insertion of three alanine residues leads to a stronger STAT activation. These results suggest that gp130-mediated activation of STATs is sensitive to rotational changes around the receptor axis perpendicular to the membrane. Surprisingly, insertion of 1, 2, 3, or 4 alanine residues into this juxtamembrane region leads to successive impairment but not abolishment of Janus kinase and receptor phosphorylation, supporting the finding of sensitivity of Janus kinases toward changes in distance of box 1/2 from the plasma membrane. We suggest a new model concerning the gp130 activation mode in which the relative orientation of the cytoplasmic regions seems to be critical for further signal transduction.

Structural requirements of the interleukin-6 signal transducer gp130 for its interaction with Janus kinase 1: the receptor is crucial for kinase activation

We analysed the interaction of gp130, the common signal-transducing receptor chain of interleukin (IL)-6 type cytokines, with Jak1, the Janus family kinase which is crucial for signal transduction of this group of cytokines. With a truncated chimaeric IL-5Rbeta-gp130 receptor expressed in COS-7 cells, we show that the membrane-proximal 69 amino acids are sufficient to mediate Jak1 binding and activation. Deletion of box2 drastically reduced binding of endogenous, but not of overexpressed, Jak1. Several point mutations in the membrane-proximal region of gp130 (W652A, P671/P672A, F676A, Y683F, where W, A, P, F and Y are tryptophan, alanine, proline, phenylalanine and tyrosine) did not affect Jak1 association. However, stimulation of chimaeric receptors with the mutations P671/P672A and F676A in the interbox1/box2 region resulted in a reduced activation of STAT (signal transducer and activator of transcription) transcription factors. Most importantly, signalling by the receptor with the box1 mutation W652A was totally abrogated. Although this mutation did not affect Jak1 association, stimulation-dependent phosphorylation of Jak1 was prevented. The W652 mutation acts dominantly, since no signalling occured even when only a single cytoplasmic chain of a gp130 dimer contained the mutation. Our data demonstrate that the mere proximity of Jaks in an activated receptor complex is not sufficient to mediate their activation. Rather, it seems that parts of the receptor, including the box1 region, are involved in positioning Jaks correctly so that ligand-induced receptor dimerization and reorientation can lead to their mutual activation and subsequently to downstream signalling events.

Mapping of a region within the N terminus of Jak1 involved in cytokine receptor interaction

Janus kinase 1 (Jak1) is a cytoplasmic tyrosine kinase that noncovalently associates with a variety of cytokine receptors. Here we show that the in vitro translated N-terminal domains of Jak1 are sufficient for binding to a biotinylated peptide comprising the membrane-proximal 73 amino acids of gp130, the signal-transducing receptor chain of interleukin-6-type cytokines. By the fold recognition approach amino acid residues 36-112 of Jak1 were predicted to adopt a beta-grasp fold, and a structural model was built using ubiquitin as a template. Substitution of Tyr(107) to alanine, a residue conserved among Jaks and involved in hydrophobic core interactions of the proposed beta-grasp domain, abrogated binding of full-length Jak1 to gp130 in COS-7 transfectants. By further mutagenesis we identified the loop 4 region of the Jak1 beta-grasp domain as essential for gp130 association and gp130-mediated signal transduction. In Jak1-deficient U4C cells reconstituted with the loop 4 Jak1 mutants L80A/Y81A and Delta(Tyr(81)-Ser(84)), the interferon-gamma, interferon-alpha, and interleukin-6 responses were similarly impaired. Thus, loop 4 of the beta-grasp domain plays a role in the association of Jak1 with both class I and II cytokine receptors. Taken together the structural model and the mutagenesis data provide further insight into the interaction of Janus kinases with cytokine receptors.

A region encompassing the FERM domain of Jak1 is necessary for binding to the cytokine receptor gp130

The terminal portion of the Janus kinases (Jaks) contains a divergent FERM (Four-point-one, Ezrin, Radixin, Moesin) homology domain comprising 19 conserved hydrophobic regions. To determine the role of this domain in governing recruitment of Jak1, but not Jak3, to the gp130 subunit of the interleukin-6 family of cytokine receptors, the interaction of three Jak1/Jak3 chimeras with gp130 was investigated. Chimeras 1, 2 and 3 (Jak1 FERM regions 1-19, 1-18 and 1-8/Jak3, respectively) were all enzymically active. Chimeras 1 and 2 interacted with the cytoplasmic domain of gp130, although less efficiently than Jak1. Only chimera 2, however, restored gp130 signalling in Jak1-negative cells. The data are consistent with recruitment of Jak1 to gp130 through the Jak1 FERM domain, but also emphasise the likely requirement for precise Jak/receptor orientation to sustain function.

Non-redundant signal transduction of interleukin-6-type cytokines. The adapter protein Shc is specifically recruited to rhe oncostatin M receptor

The common use of the cytokine receptor gp130 has served as an explanation for the extremely redundant biological activities exerted by interleukin (IL)-6-type cytokines. Indeed, hardly any differences in signal transduction initiated by these cytokines are known. In the present study, we demonstrate that oncostatin M (OSM), but not IL-6 or leukemia inhibitory factor, induces tyrosine phosphorylation of the Shc isoforms p52 and p66 and their association with Grb2. Concomitantly, OSM turns out to be a stronger activator of ERK1/2 MAPKs. Shc is recruited to the OSM receptor (OSMR), but not to gp130. Binding involves Tyr(861) of the OSMR, located within a consensus binding sequence for the Shc PTB domain. Moreover, Tyr(861) is essential for activation of ERK1/2 and for full activation of the alpha(2)-macroglobulin promoter, but not for an exclusively STAT-responsive promoter. This study therefore provides evidence for qualitative differential signaling mechanisms exerted by IL-6-type cytokines.

The cytoplasmic domain of the interleukin-6 receptor gp80 mediates its basolateral sorting in polarized madin-darby canine kidney cells

The IL-6 receptor complex is expressed in different polarized epithelial cells such as liver hepatocytes and intestinal cells. It consists of two subunits: gp80, which binds the ligand, and gp130, which is responsible for signal transduction. In stably transfected Madin-Darby canine kidney (MDCK) cells we have studied the localization of the human IL-6 receptor subunits and found that gp80 and gp130 are predominantly expressed at the basolateral membrane. Analysis of MDCK cells expressing truncated forms of gp80 or gp130 showed that loss of the cytoplasmic domains results in apical delivery. Expression of deletion mutants of gp80 in MDCK cells led to the identification of two discontinous motifs responsible for basolateral sorting: a membrane-proximal tyrosine-based motif (YSLG) and a more membrane-distal dileucine-type motif (LI). Activation of signal transducer and activator of transcription-3 (STAT-3) only occurred via basolaterally located gp80, suggesting that endogenous gp130 is also constrained to the basolateral plasma membrane. Our identification of a basolateral sorting signal within the cytoplasmic region of gp80 for the first time attributes a function to this domain.

Signal transduction of IL-6, leukemia-inhibitory factor, and oncostatin M: structural receptor requirements for signal attenuation

Stimulation of the IL-6R complex leads to Src homology domain containing tyrosine phosphatase 2 (SHP2) recruitment to the receptor subunit gp130 and its subsequent tyrosine phosphorylation. SHP2 is a two-SH2 domain-containing protein tyrosine phosphatase that is activated by many cytokines and growth factors. SHP2 counteracts the activation of transcription factors of the STAT family and the induction of IL-6-responsive genes. Tyrosine 759 of gp130, the signal transducing subunit of the IL-6R complex, is essential for the phosphorylation of SHP2. Mutation of tyrosine 759 to phenylalanine leads to an enhanced inducibility of IL-6-dependent genes. Here we demonstrate that no further tyrosines in the cytoplasmic part of gp130 are required for the phosphorylation of SHP2. We also tested whether the tyrosine 759 motifs in both subunits of the gp130 dimer are required for SHP2 association and tyrosine phosphorylation. Interestingly, one SHP2-recruiting phosphotyrosine motif in a single chain of the gp130 dimer is sufficient to mediate SHP2 association to the gp130 receptor subunit and its tyrosine phosphorylation as well as to attenuate IL-6-dependent gene induction. Furthermore, we show that repression of gene induction via Y759 does not require the presence of the SHP2 and STAT recruitment sites within the same receptor subunit, but within the same receptor complex. The Y759 motif in gp130 also attenuates gene induction mediated by the oncostatin M and leukemia inhibitory factor receptor complexes, which both contain gp130 as the shared subunit.

Differential inhibition of IL-6-type cytokine-induced STAT activation by PMA

Prior activation of mitogen-activated protein kinases by phorbol 13-myristate 12-acetate (PMA) results in an inhibition of interleukin (IL)-6-induced activation of the Janus kinase/signal transducer and activator of transcription (STAT) signaling pathway which is most likely mediated by the induction of suppressor of cytokine signaling-3 and requires the specific SHP2 binding site Y759 of the IL-6 signal transducer gp130. In this study, we demonstrate that PMA inhibits STAT activation by IL-6 and the related cytokine leukemia inhibitory factor (LIF) but not by oncostatin M (OSM). Since the LIF receptor also contains an SHP2 recruitment site whereas the OSM receptor lacks such a module, we propose that two SHP2 binding modules within a homo- or heterodimeric receptor are necessary to mediate the PMA inhibitory effect.

A single amino acid substitution (Trp(666)-->Ala) in the interbox1/2 region of the interleukin-6 signal transducer gp130 abrogates binding of JAK1, and dominantly impairs signal transduction

gp130 is the common signal-transducing receptor chain of interleukin (IL)-6-type cytokines. Here we describe, for the first time, a single amino acid substitution (Trp(666)-->Ala) in the membrane-proximal interbox1/2 region that abrogates activation of STAT (signal transducer and activator of transcription) transcription factors and the proliferative response of pro-B-cell transfectants. Moreover, association of the Janus kinase JAK1 is prevented. No signalling of heterodimeric IL-5 receptor (IL-5R)/gp130 chimaeras occurs in COS-7 cells, even when only a single cytoplasmic chain of a gp130 dimer contains the Trp(666)Ala mutation, indicating that it acts dominantly.

Molecular cloning of a novel type 1 cytokine receptor similar to the common gamma chain

In a complementary DNA (cDNA) screening of murine Th2-skewed lymphocytes with our recently developed signal sequence trap method termed SST-REX, a novel type 1 cytokine receptor, Delta1 (δ1), was identified. Although δ1 is ubiquitously expressed in multiple tissues, the expression level is higher in Th2-skewed lymphocytes than in Th1-skewed ones. The δ1 cDNA encodes a 359–amino acid type 1 membrane protein. The extracellular domain of 206 amino acids showed 24% identity with the murine common γ receptor that is shared among the receptors for interleukin(IL)-2, IL-4, IL-7, IL-9, and IL-15. The membrane-proximal region of δ1 includes a box1 motif, which is important for association with Janus kinases (JAKs), and showed a significant homology with that of the mouse erythropoietin receptor (EPOR). A box2 motif was also found in close proximity to the box1 region. Dimerization of the cytoplasmic region of δ1 alone did not transduce proliferative signals in IL-3–dependent cell lines. However, the membrane-proximal region of δ1 could substitute for that of human EPOR in transmitting proliferative signals and activating JAK2. These results suggest that δ1 is a subunit of cytokine receptor that may be involved in multiple receptor systems and play a regulatory role in the immune system and hematopoiesis.

Contributions of Leukemia Inhibitory Factor Receptor and Oncostatin M Receptor to Signal Transduction in Heterodimeric Complexes with Glycoprotein 130

Leukemia inhibitory factor (LIF), cardiotrophin-1, ciliary neurotrophic factor, and oncostatin M (OSM) lead to heterodimerization of LIF receptor (LIFR) or the OSM-specific receptor (OSMR) with glycoprotein (gp) 130, the common receptor subunit for IL-6-type cytokines. Thereby intracellular signaling via Janus kinases (Jaks) and STAT transcription factors is initiated. We investigated the contributions of LIFR and OSMR to signal transduction in the context of heterodimers with gp130. Chimeric receptors based on the extracellular parts of the IL-5R α- and β-chains were generated, allowing the induced heterodimerization of two different cytoplasmic tails. Our studies demonstrate that upon heterodimerization with the gp130 cytoplasmic region, the cytoplasmic parts of both LIFR and OSMR were critical for activation of an acute phase protein promoter in HepG2 hepatoma cells. The membrane-proximal region of LIFR or OSMR was crucial for the ability of such receptor complexes to induce DNA binding of STAT1 and STAT3 in COS-7 cells. Membrane-distal regions of LIFR and OSMR contributed to STAT activation even in the absence of gp130 STAT recruitment sites. We further show that the Janus kinases Jak1 and Jak2 constitutively associated with receptor constructs containing the cytoplasmic part of LIFR, OSMR, or gp130, respectively. Homodimers of the LIFR or OSMR cytoplasmic regions did not elicit responses in COS-7 cells but did in HepG2 cells and in MCF-7 breast carcinoma cells. Thus, in spite of extensive functional similarities, differential signaling abilities of gp130, LIFR, and OSMR may become evident in a cell-type-specific manner.

Dimerization of the interferon type I receptor IFNaR2-2 is sufficient for induction of interferon effector genes but not for full antiviral activity

We constructed chimeric receptors wherein the extracellular domain of the erythropoietin receptor (EpoR) was fused to the transmembrane and intracellular domains of the interferon (IFN) type I receptor subunits, IFNaR1 or IFNaR2-2. Transfection into 2fTGH and Tyk2-deficient 11,1 cells showed that EpoR/IFNaR2-2 alone was able to transduce a signal upon stimulation with erythropoietin (Epo), as judged by induction of the interferon type I-inducible 6-16 promoter. In contrast, protection against infection with encephalomyocarditis virus or vesicular stomatitis virus was reduced or absent, respectively. To further investigate the role of IFNaR1 in the induction of an antiviral state, we analyzed the Epo- versus IFNalpha-induced transcription of a set of genes, involved in antiviral protection. Up to 24 h after stimulation with Epo or IFNalpha, comparable transcription of the p56, dsRNA-dependent protein kinase, 2'-5'A synthetase, and MxA genes was seen. However, at later time points, only in the case of Epo induction, a sharp decrease of mRNA levels was observed. Western blotting analysis of dsRNA-dependent protein kinase showed a similar pattern at the protein level. Taken together, our results imply a role for IFNaR1 in the induction of sustained mRNA and protein levels that are likely required for optimal antiviral activity.

The cell surface expression level of the human interleukin-5 receptor alpha subunit determines the agonistic/antagonistic balance of the human interleukin-5 E13Q mutein

The human interleukin-5 (IL-5) receptor consists of an alpha-chain that specifically binds the ligand with intermediate affinity, and a beta c-chain, that associates with the IL-5/IL-5R alpha complex, leading to a high-affinity, signal transducing receptor complex. Structure-function studies showed that modification of the putative beta c-chain binding site in IL-5 (E13Q mutein) converted the molecule into an antagonist. However, analysis of the effect of this mutant IL-5 on COS-1 cells transfected with both receptor subunits, did not show reduced interaction with the beta c subunit [Tavernier, J., Tuypens, T., Verhee, A., Plaetinck, G., Devos, R., Van der Heyden, J., Guisez, Y. & Oefner, C. (1995) Proc. Natl Acad. Sci. USA 89, 7041-7045]. To gain more insight into the mechanism of IL-5 antagonism by E13Q, we tested its biological activity on two FDC-P1 subclones that express clearly different numbers of alpha-subunits yet an almost constant number of murine beta c-subunits. Here we show that E13Q has a biological activity comparable to wild-type IL-5 only when a high number of alpha-chains is present on the cells. Confirming the critical role of the IL5R alpha cell-surface expression level, treatment with suboptimal doses of a neutralising anti-IL-5R alpha antibody results in reduced activity of the mutant but not of wild-type IL-5.

Differential substrate recognition capabilities of Janus family protein tyrosine kinases within the interleukin 2 receptor (IL2R) system: Jak3 as a potential molecular target for treatment of leukemias with a hyperactive Jak-Stat signaling machinery

Substrate recognition by Janus family protein tyrosine kinases was examined utilizing recombinant baculovirus produced components of the interleukin 2 receptor (IL2R) system i.e. Jak1, Transducers and Activators of Transcription (STAT). Wild type Jak3 was able to tyrosine phosphorylate a kinase-dead Jak1 (Jak1E908). In contrast wild type Jak1 was unable to tyrosine phosphorylate kinase dead Jak3 (Jak3E851). This unilateral transphosphorylation between Jak3 and Jak1 prompts the hypothesis that in the IL2R system the activation of Jak3 precedes Jak1 activation. Both the IL2Rbeta and IL2Rgammac subunits underwent tyrosine phosphorylation when co-expressed with wild-type Jak3. By comparison only IL2Rbeta was recognized and tyrosine phosphorylated by wild-type Jak1. These results are consistent with the notion that Jakl is pre-associated with IL2Rbeta and Jak3 is pre-associated with IL2Rgammac. STAT1, STAT3, and STAT5 underwent tyrosine phosphorylation when co-expressed with Jakl and therefore are substrates for the respective Jak kinases. In contrast, Jak3 co-expression resulted in tyrosine phosphorylation of STAT3 and STAT5 but not STAT1. Notably a polypeptide representing the kinase domain of Jak3 (Jak3-JH1) gained the ability to tyrosine phosphorylate STAT1, suggesting that the changes in substrate recognition may be influenced by domains outside the kinase domain. These findings provide evidence that Jak1 and Jak3 differentially recognize specific substrates, thereby having the ability to contribute specific signals, and the substrate specificity may be influenced by multiple domains of these tyrosine kinases.

Interleukin-6-type cytokine signalling through the gp130/Jak/STAT pathway

The family of cytokines signalling through the common receptor subunit gp130 comprises interleukin (IL)-6, IL-11, leukaemia inhibitory factor, oncostatin M, ciliary neurotrophic factor and cardiotrophin-1. These so-called IL-6-type cytokines play an important role in the regulation of complex cellular processes such as gene activation, proliferation and differentiation. The current knowledge on the signal-transduction mechanisms of these cytokines from the plasma membrane to the nucleus is reviewed. In particular, we focus on the assembly of receptor complexes after ligand binding, the activation of receptor-associated kinases of the Janus family, and the recruitment and phosphorylation of transcription factors of the STAT family, which dimerize, translocate to the nucleus, and bind to enhancer elements of respective target genes leading to transcriptional activation. The important players in the signalling pathway, namely the cytokines and the receptor components, the Janus kinases Jak1, Jak2 and Tyk2, the signal transducers and activators of transcription STAT1 and STAT3 and the tyrosine phosphatase SHP2 [SH2 (Src homology 2) domain-containing tyrosine phosphatase] are introduced and their structural/functional properties are discussed. Furthermore, we review various mechanisms involved in the termination of the IL-6-type cytokine signalling, namely the action of tyrosine phosphatases, proteasome, Jak kinase inhibitors SOCS (suppressor of cytokine signalling), protein inhibitors of activated STATs (PIAS), and internalization of the cytokine receptors via gp130. Although all IL-6-type cytokines signal through the gp130/Jak/STAT pathway, the comparison of their physiological properties shows that they elicit not only similar, but also distinct, biological responses. This is reflected in the different phenotypes of IL-6-type-cytokine knock-out animals.

Dimerization as a regulatory mechanism in signal transduction

Dynamic protein-protein interactions are a key component of biological regulatory networks. Dimerization events--physical interactions between related proteins--represent an important subset of protein-protein interactions and are frequently employed in transducing signals from the cell surface to the nucleus. Importantly, dimerization between different members of a protein family can generate considerable functional diversity when different protein combinations have distinct regulatory properties. A survey of processes known to be controlled by dimerization illustrates the diverse physical and biological outcomes achieved through this regulatory mechanism. These include: facilitated proximity and orientation; differential regulation by heterodimerization; generation of temporal and spatial boundaries; enhancement of specificity; and regulated monomer-to-dimer transitions. Elucidation of these mechanisms has led to the design of new approaches to study and to manipulate signal transduction pathways.

Protein tyrosine phosphatase 2 (SHP-2) moderates signaling by gp130 but is not required for the induction of acute-phase plasma protein genes in hepatic cells

Signals propagated via the gp130 subunit of the interleukin-6 (IL-6)-type cytokine receptors mediate, among various cellular responses, proliferation of hematopoietic cells and induction of acute-phase plasma protein (APP) genes in hepatic cells. Hematopoietic growth control by gp130 is critically dependent on activation of both STAT3 and protein tyrosine phosphatase 2 (SHP-2). To investigate whether induction of APP genes has a similar requirement for SHP-2, we constructed two chimeric receptors, G-gp130 and G-gp130(Y2F), consisting of the transmembrane and cytoplasmic domains of gp130 harboring either a wild-type or a mutated SHP-2 binding site, respectively, fused to the extracellular domain of the granulocyte colony-stimulating factor (G-CSF) receptor. Rat hepatoma H-35 cells stably expressing the chimeric receptors were generated by retroviral transduction. Both chimeric receptors transmitted a G-CSF-induced signal characteristic of that triggered by IL-6 through the endogenous gp130 receptor; i.e., both activated the appropriate JAK, induced DNA binding activity by STAT1 and STAT3, and up-regulated expression of the target APP genes, those for alpha-fibrinogen and haptoglobin. Notwithstanding these similarities in the patterns of signaling responses elicited, mutation of the SHP-2 interaction site in G-gp130(Y2F) abrogated ligand-activated receptor recruitment of SHP-2 as expected. Moreover, the tyrosine phosphorylation state of the chimeric receptor, the associated JAK activity, and the induced DNA binding activity of STAT1 and STAT3 were maintained at elevated levels and for an extended period of time in G-gp130(Y2F)-expressing cells following G-CSF treatment compared to that in cells displaying the G-gp130 receptor. H-35 cells ectopically expressing G-gp130(Y2F) were also found to display an enhanced sensitivity to G-CSF and a higher level of induction of APP genes. Overexpression of the enzymatically inactive SHP-2 enhanced the signaling by the wild-type but not by the Y2F mutant G-gp130 receptor. These results indicate that gp130 signaling for APP gene induction in hepatic cells differs qualitatively from that controlling the proliferative response in hematopoietic cells in not being strictly dependent on SHP-2. The data further suggest that SHP-2 functions normally to attenuate gp130-mediated signaling in hepatic (and, perhaps, other) cells by moderating JAK action.

Internalization of the interleukin 6 signal transducer gp130 does not require activation of the Jak/STAT pathway

Signalling receptors often undergo receptor-mediated endocytosis. In many cases this internalization is stimulated by ligand binding and activation of intrinsic receptor tyrosine kinases, resulting in a receptor down-regulation. We have analysed whether internalization of the interleukin 6 signal transducer gp130 is dependent on the activation of receptor-associated Jak kinases. By using a chimaeric receptor system we found that receptor mutants that lack box1 and therefore are not capable of activating Jak and signal transducer and activator of transcription (STAT) proteins are still endocytosed efficiently. A chimaeric receptor with the recently identified dileucine internalization motif being replaced by two alanine residues was not efficiently internalized but still capable of recruiting STATs. Furthermore an antagonistic antibody that inhibits the signalling of all interleukin-6-type cytokines via gp130 was internalized as efficiently as an agonistic one that activates the Jak/STAT pathway. Our findings suggest that the endocytosis of gp130 is signal-independent.

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.