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

JAK-STAT signaling in inflammation and stress-related diseases: implications for therapeutic interventions

The Janus kinase-signal transducer and transcription activator pathway (JAK-STAT) serves as a cornerstone in cellular signaling, regulating physiological and pathological processes such as inflammation and stress. Dysregulation in this pathway can lead to severe immunodeficiencies and malignancies, and its role extends to neurotransduction and pro-inflammatory signaling mechanisms. Although JAK inhibitors (Jakinibs) have successfully treated immunological and inflammatory disorders, their application has generally been limited to diseases with similar pathogenic features. Despite the modest expression of JAK-STAT in the CNS, it is crucial for functions in the cortex, hippocampus, and cerebellum, making it relevant in conditions like Parkinson's disease and other neuroinflammatory disorders. Furthermore, the influence of the pathway on serotonin receptors and phospholipase C has implications for stress and mood disorders. This review expands the understanding of JAK-STAT, moving beyond traditional immunological contexts to explore its role in stress-related disorders and CNS function. Recent findings, such as the effectiveness of Jakinibs in chronic conditions such as rheumatoid arthritis, expand their therapeutic applicability. Advances in isoform-specific inhibitors, including filgotinib and upadacitinib, promise greater specificity with fewer off-target effects. Combination therapies, involving Jakinibs and monoclonal antibodies, aiming to enhance therapeutic specificity and efficacy also give great hope. Overall, this review bridges the gap between basic science and clinical application, elucidating the complex influence of the JAK-STAT pathway on human health and guiding future interventions.

Competitive binding of STATs to receptor phospho-Tyr motifs accounts for altered cytokine responses

Cytokines elicit pleiotropic and non-redundant activities despite strong overlap in their usage of receptors, JAKs and STATs molecules. We use IL-6 and IL-27 to ask how two cytokines activating the same signaling pathway have different biological roles. We found that IL-27 induces more sustained STAT1 phosphorylation than IL-6, with the two cytokines inducing comparable levels of STAT3 phosphorylation. Mathematical and statistical modeling of IL-6 and IL-27 signaling identified STAT3 binding to GP130, and STAT1 binding to IL-27Rα, as the main dynamical processes contributing to sustained pSTAT1 levels by IL-27. Mutation of Tyr613 on IL-27Rα decreased IL-27-induced STAT1 phosphorylation by 80% but had limited effect on STAT3 phosphorgylation. Strong receptor/STAT coupling by IL-27 initiated a unique gene expression program, which required sustained STAT1 phosphorylation and IRF1 expression and was enriched in classical Interferon Stimulated Genes. Interestingly, the STAT/receptor coupling exhibited by IL-6/IL-27 was altered in patients with systemic lupus erythematosus (SLE). IL-6/IL-27 induced a more potent STAT1 activation in SLE patients than in healthy controls, which correlated with higher STAT1 expression in these patients. Partial inhibition of JAK activation by sub-saturating doses of Tofacitinib specifically lowered the levels of STAT1 activation by IL-6. Our data show that receptor and STATs concentrations critically contribute to shape cytokine responses and generate functional pleiotropy in health and disease.

IL-6 in inflammation, autoimmunity and cancer

IL-6 is involved both in immune responses and in inflammation, hematopoiesis, bone metabolism and embryonic development. IL-6 plays roles in chronic inflammation (closely related to chronic inflammatory diseases, autoimmune diseases and cancer) and even in the cytokine storm of corona virus disease 2019 (COVID-19). Acute inflammation during the immune response and wound healing is a well-controlled response, whereas chronic inflammation and the cytokine storm are uncontrolled inflammatory responses. Non-immune and immune cells, cytokines such as IL-1β, IL-6 and tumor necrosis factor alpha (TNFα) and transcription factors nuclear factor-kappa B (NF-κB) and signal transducer and activator of transcription 3 (STAT3) play central roles in inflammation. Synergistic interactions between NF-κB and STAT3 induce the hyper-activation of NF-κB followed by the production of various inflammatory cytokines. Because IL-6 is an NF-κB target, simultaneous activation of NF-κB and STAT3 in non-immune cells triggers a positive feedback loop of NF-κB activation by the IL-6-STAT3 axis. This positive feedback loop is called the IL-6 amplifier (IL-6 Amp) and is a key player in the local initiation model, which states that local initiators, such as senescence, obesity, stressors, infection, injury and smoking, trigger diseases by promoting interactions between non-immune cells and immune cells. This model counters dogma that holds that autoimmunity and oncogenesis are triggered by the breakdown of tissue-specific immune tolerance and oncogenic mutations, respectively. The IL-6 Amp is activated by a variety of local initiators, demonstrating that the IL-6-STAT3 axis is a critical target for treating diseases.

Dominant-negative mutations in human IL6ST underlie hyper-IgE syndrome

Autosomal dominant hyper-IgE syndrome (AD-HIES) is typically caused by dominant-negative (DN) STAT3 mutations. Patients suffer from cold staphylococcal lesions and mucocutaneous candidiasis, severe allergy, and skeletal abnormalities. We report 12 patients from 8 unrelated kindreds with AD-HIES due to DN IL6ST mutations. We identified seven different truncating mutations, one of which was recurrent. The mutant alleles encode GP130 receptors bearing the transmembrane domain but lacking both the recycling motif and all four STAT3-recruiting tyrosine residues. Upon overexpression, the mutant proteins accumulate at the cell surface and are loss of function and DN for cellular responses to IL-6, IL-11, LIF, and OSM. Moreover, the patients' heterozygous leukocytes and fibroblasts respond poorly to IL-6 and IL-11. Consistently, patients with STAT3 and IL6ST mutations display infectious and allergic manifestations of IL-6R deficiency, and some of the skeletal abnormalities of IL-11R deficiency. DN STAT3 and IL6ST mutations thus appear to underlie clinical phenocopies through impairment of the IL-6 and IL-11 response pathways.

SHP-2 in Lymphocytes' Cytokine and Inhibitory Receptor Signaling

Somewhat counterintuitively, the tyrosine phosphatase SHP-2 (SH2 domain-containing protein tyrosine phosphatase-2) is crucial for the activation of extracellular signal-regulated kinase (ERK) downstream of various growth factor receptors, thereby exerting essential developmental functions. This phosphatase also deploys proto-oncogenic functions and specific inhibitors have recently been developed. With respect to the immune system, the role of SHP-2 in the signaling of cytokines relevant for myelopoiesis and myeloid malignancies has been intensively studied. The function of this phosphatase downstream of cytokines important for lymphocytes is less understood, though multiple lines of evidence suggest its importance. In addition, SHP-2 has been proposed to mediate the suppressive effects of inhibitory receptors (IRs) that sustain a dysfunctional state in anticancer T cells. Molecules involved in IR signaling are of potential pharmaceutical interest as blockade of these inhibitory circuits leads to remarkable clinical benefit. Here, we discuss the dichotomy in the functions ascribed to SHP-2 downstream of cytokine receptors and IRs, with a focus on T and NK lymphocytes. Further, we highlight the importance of broadening our understanding of SHP-2′s relevance in lymphocytes, an essential step to inform on side effects and unanticipated benefits of its therapeutic blockade.

Is there a role for prostanoid-mediated inhibition of IL-6 trans-signalling in the management of pulmonary arterial hypertension?

Inflammation has been highlighted as a key factor in pulmonary arterial hypertension (PAH) development, particularly interleukin-6 (IL-6). IL-6 activates JAK-STAT signalling to induce transcription of pro-inflammatory and pro-angiogenic genes, enabling PAH progression, as well as the transcription of suppressor of cytokine signalling 3 (SOCS3) which limits IL-6 signalling. Current PAH therapies include prostanoid drugs which induce vasodilation via stimulating intracellular 3',5'-cyclic adenosine monophosphate (cAMP) levels. cAMP can also inhibit IL-6-mediated endothelial dysfunction via the induction of SOCS3. Thus, we propose that an important mechanism by which cAMP-mobilising prostanoid drugs limit PAH is by inhibiting IL-6-mediated pulmonary inflammation and remodelling via SOCS3 inhibition of IL-6 signalling. Further clarification may result in effective strategies with which to target the IL-6/JAK-STAT signalling pathway in PAH.

Intragenic regulation of SOCS3 isoforms

Background

Inflammatory reactions are commonly affected by stress responses. Interleukin-6 signalling is part of the inflammatory response and is stringently regulated by the feedback inhibitor SOCS3 expressed in a short and long isoform. Here, we studied the inhibitory potential of the two SOCS3 isoforms. Furthermore, we analysed the regulation of SOCS3 isoform expression and the role of PKR stress kinase signalling in SOCS3 protein expression.

Methods

We performed Western blotting, reporter assays, genetic analyses and manipulations for studying SOCS3 isoform expression and activation of signalling components involved in interleukin-6-induced and PKR-dependent signalling.

Results

Interleukin-6-induced endogenous expression of both SOCS3 isoforms was found in distinct cell types. Forced expression of either the long or short SOCS3 isoform demonstrated equal inhibitory activity of each isoform and confirmed longer half-life of the short isoform. Study of intragenic regulation of SOCS3 isoform expression revealed that (i) the 5′-UTR of SOCS3 mRNA restrains specifically expression of the long SOCS3 isoform, (ii) expression of the long isoform restrains expression of the short isoform, and (iii) signalling through the stress kinase PKR does not impact on SOCS3 isoform ratio.

Conclusions

Both SOCS3 isoforms show a similar potential for inhibiting interleukin-6 signalling but differ in their half-lives. Relative expression of the isoforms depends on intragenic elements yet is independent of PKR signalling.

Graphic abstract

Electronic supplementary material

The online version of this article (10.1186/s12964-019-0379-6) contains supplementary material, which is available to authorized users.

Efficacy, Safety, Pharmacokinetics, and Pharmacodynamics of Filgotinib, a Selective JAK-1 Inhibitor, After Short-Term Treatment of Rheumatoid Arthritis: Results of Two Randomized Phase IIa Trials

OBJECTIVE

JAK inhibitors have shown efficacy in rheumatoid arthritis (RA). We undertook this study to test our hypothesis that selective inhibition of JAK-1 would combine good efficacy with a better safety profile compared with less selective JAK inhibitors.

METHODS

In two 4-week exploratory, double-blind, placebo-controlled phase IIa trials, 127 RA patients with an insufficient response to methotrexate (MTX) received filgotinib (GLPG0634, GS-6034) oral capsules (100 mg twice daily or 30, 75, 150, 200, or 300 mg once daily) or placebo, added onto a stable regimen of MTX, to evaluate safety, efficacy, pharmacokinetics (PK), and pharmacodynamics (PD) of filgotinib. The primary efficacy end point was the number and percentage of patients in each treatment group meeting the American College of Rheumatology 20% improvement criteria (achieving an ACR20 response) at week 4.

RESULTS

Treatment with filgotinib at 75-300 mg met the primary end point and showed early onset of efficacy. ACR20 response rates progressively increased to week 4, and the Disease Activity Score in 28 joints using the C-reactive protein (CRP) level decreased. Marked and sustained improvements were observed in serum CRP level and other PD markers. The PK of filgotinib and its major metabolite was dose proportional over the 30-300 mg range. Early side effects seen with other less selective JAK inhibitors were not observed (e.g., there was no worsening of anemia [JAK-2 inhibition related], no effects on liver transaminases, and no increase in low-density lipoprotein or total cholesterol). A limited decrease in neutrophils without neutropenia was consistent with immunomodulatory effects through JAK-1 inhibition. There were no infections. Overall, filgotinib was well tolerated. Events related to study drug were mild or moderate and transient during therapy, and the most common such event was nausea.

CONCLUSION

Selective inhibition of JAK-1 with filgotinib shows initial efficacy in RA with an encouraging safety profile in these exploratory studies.

The tyrosine kinase inhibitor nintedanib activates SHP-1 and induces apoptosis in triple-negative breast cancer cells

Triple-negative breast cancer (TNBC) remains difficult to treat and urgently needs new therapeutic options. Nintedanib, a multikinase inhibitor, has exhibited efficacy in early clinical trials for HER2-negative breast cancer. In this study, we examined a new molecular mechanism of nintedanib in TNBC. The results demonstrated that nintedanib enhanced TNBC cell apoptosis, which was accompanied by a reduction of p-STAT3 and its downstream proteins. STAT3 overexpression suppressed nintedanib-mediated apoptosis and further increased the activity of purified SHP-1 protein. Moreover, treatment with either a specific inhibitor of SHP-1 or SHP-1-targeted siRNA reduced the apoptotic effects of nintedanib, which validates the role of SHP-1 in nintedanib-mediated apoptosis. Furthermore, nintedanib-induced apoptosis was attenuated in TNBC cells expressing SHP-1 mutants with constantly open conformations, suggesting that the autoinhibitory mechanism of SHP-1 attenuated the effects of nintedanib. Importantly, nintedanib significantly inhibited tumor growth via the SHP-1/p-STAT3 pathway. Clinically, SHP-1 levels were downregulated, whereas p-STAT3 was upregulated in tumor tissues, and SHP-1 transcripts were associated with improved disease-free survival in TNBC patients. Our findings revealed that nintedanib induces TNBC apoptosis by acting as a SHP-1 agonist, suggesting that targeting STAT3 by enhancing SHP-1 expression could be a viable therapeutic strategy against TNBC.

Stress-induced dynamic regulation of mitochondrial STAT3 and its association with cyclophilin D reduce mitochondrial ROS production

Signal transducer and activator of transcription 3 (STAT3) is associated with various physiological and pathological functions, mainly as a transcription factor that translocates to the nucleus upon tyrosine phosphorylation induced by cytokine stimulation. In addition, a small pool of STAT3 resides in the mitochondria, where it serves as a sensor for various metabolic stressors including reactive oxygen species (ROS). Mitochondrially localized STAT3 largely exerts its effects through direct or indirect regulation of the activity of the electron transport chain (ETC). It has been assumed that the amounts of STAT3 in the mitochondria are static. We showed that various stimuli, including oxidative stress and cytokines, triggered a signaling cascade that resulted in a rapid loss of mitochondrially localized STAT3. Recovery of the mitochondrial pool of STAT3 over time depended on phosphorylation of Ser727 in STAT3 and new protein synthesis. Under these conditions, mitochondrially localized STAT3 also became competent to bind to cyclophilin D (CypD). Binding of STAT3 to CypD was mediated by the amino terminus of STAT3, which was also important for reducing mitochondrial ROS production after oxidative stress. These results outline a role for mitochondrially localized STAT3 in sensing and responding to external stimuli.

Shp2 Inhibits Proliferation of Esophageal Squamous Cell Cancer via Dephosphorylation of Stat3

Shp2 (Src-homology 2 domain-containing phosphatase 2) was originally reported as an oncogene in kinds of solid tumors and hematologic malignancies. However, recent studies indicated that Shp2 may act as tumor suppressors in several tumor types. We investigated the function of Shp2 in esophageal squamous cell cancer (ESCC). The expression level of Shp2 was analyzed in tumor tissues in comparison with adjacent normal tissues of ESCC patients by immunohistochemistry and Western blot. Shp2 was knocked down by Short hairpin RNA to evaluate its function in ESCC cell lines. The relationship between Shp2 and p-Stat3 (signal transducer and activator of transcription 3) in human ESCC tissues was statistically examined. A significant low expression of Shp2 was found in ESCC tissues. Low expression of Shp2 was related to poorer overall survival in patients from The Cancer Genome Atlas (TCGA) dataset. Knockdown of Shp2 increased the growth of ESCC cell lines both in vivo and vitro. Activation of Stat3 (p-Stat3) was induced by Shp2 depletion. Expression of p-Stat3 was negatively correlated with Shp2 expression in ESCC tissues. Furthermore, knockdown of Shp2 attenuated cisplatin-sensitivity of ESCC cells. Shp2 might suppress the proliferation of ESCC by dephosphorylation of p-Stat3 and represents a novel research field for targeted therapy.

Ginkgetin Blocks Constitutive STAT3 Activation and Induces Apoptosis through Induction of SHP-1 and PTEN Tyrosine Phosphatases

Ginkgetin, a biflavone from Ginkgo biloba leaves, is known to exhibit antiinflammatory, antifungal, neuroprotective, and antitumor activities, but its precise mechanism of action has not been fully elucidated. Because the aberrant activation of STAT3 has been linked with regulation of inflammation, proliferation, invasion, and metastasis of tumors, we hypothesized that ginkgetin modulates the activation of STAT3 in tumor cells. We found that ginkgetin clearly suppressed constitutive phosphorylation of STAT3 through inhibition of the activation of upstream JAK1 and c-Src kinases and nuclear translocation of STAT3 on both A549 and FaDu cells. Treatment with sodium pervanadate reversed the ginkgetin-induced down-modulation of STAT3, thereby indicating a critical role for a PTP. We also found that ginkgetin strongly induced the expression of the SHP-1 and PTEN proteins and its mRNAs. Further, deletion of SHP-1 and PTEN genes by siRNA suppressed the induction of SHP-1 and PTEN, and reversed the inhibition of STAT3 activation. Ginkgetin induced apoptosis as characterized by an increased accumulation of cells in subG1 phase, positive Annexin V binding, loss of mitochondrial membrane potential, down-regulation of STAT3-regulated gene products, and cleavage of PARP. Overall, ginkgetin abrogates STAT3 signaling pathway through induction of SHP-1 and PTEN proteins, thus attenuating STAT3 phosphorylation and tumorigenesis.

Human β-defensin 3 induces STAT1 phosphorylation, tyrosine phosphatase activity, and cytokine synthesis in T cells

The AMP hBD-3 stimulates numerous immune effector functions in myeloid cells and keratinocytes, predominantly through the MAPK signaling cascade. In contrast, hBD-3 was reported to neutralize the activation of T cells by antagonizing MAPK signaling initiated by SDF-1α through CXCR4. With the use of complementary proteomic and immunochemical approaches, we investigated possible stimulatory effects of hBD-3 on T cells and demonstrate that hBD-3 induces STAT1 tyrosine phosphorylation within 5 min yet is unable to induce MAPK activation. Inclusion of a PTPase inhibitor increased hBD-3-induced phosphorylation dramatically, suggesting that hBD-3 also stimulates PTPase activity concurrently. The increase in PTPase activity was confirmed by demonstrating that hBD-3 suppresses IFN-γ-induced STAT1 tyrosine phosphorylation but not STAT1 serine and ERK1/2 threonine phosphorylation and stimulates the translocation of SHP-2 into the nucleus within 15 min. The signaling pathways initiated by hBD-3 may lead to the observed enhancement of distinct T cell effector functions during TCR activation, such as the increase in IL-2 and IL-10, but not IFN-γ secretion. Thus, hBD-3 initiates distinct lineage-specific signaling cascades in various cells involved in host defense and induces a concurrent tyrosine kinase and tyrosine phosphatase signaling cascade that may activate simultaneously the targeted T cells and inhibit their response to other immune mediators. Furthermore, these results suggest that this evolutionarily conserved peptide, which exhibits a broad spectrum of antimicrobial and immunomodulatory activities, serves to integrate innate and adaptive immunity.

Determination of protein turnover rates in the JAK/STAT pathway using a radioactive pulse-chase approach

The turnover rate of different protein species in a signal transduction network strongly affects the impact of the given species on the outcome of a stimulus. Whereas stable, long-lived proteins mainly account for the transmission of a signal, unstable short-lived species often comprise regulatory functions. Here, we describe a method to determine the half-lives of proteins of the JAK/STAT pathway by a pulse-chase approach in cell culture. First, radioactive labeling with (35)S-methionine is carried out to label newly synthesized proteins (pulse). Subsequently, the dynamics of the decay of these proteins is monitored in the absence of labeled amino acids over a defined time period (chase). For this purpose the protein of interest is isolated by immunoprecipitation from total cell lysates, separated on an SDS-polyacrylamide gel, and subsequently visualized by autoradiography.

Loss and dysregulation of Th17 cells during HIV infection

Bacterial translocation across the damaged mucosal epithelium has emerged as a major paradigm for chronic immune activation observed during HIV infection. T helper 17 (Th17) cells are a unique lineage of T helper cells that are enriched in mucosal tissues and are thought to play a central role in protecting the integrity of the mucosal barrier and maintaining immune homeostasis at mucosal sites. Th17 cells are lost very early during the course of HIV infection, and their loss has been shown to correlate with bacterial translocation. Interestingly, Th17 cells are unable to completely recover from the early destruction even after successful antiretroviral therapy (ART). Here, we review some of the potential mechanisms for the loss and dysregulation of Th17 cells during HIV infection.

Dynamic regulation of Bcl6 in follicular helper CD4 T (Tfh) cells

Our bodies are continuously exposed to various types of infectious pathogens. Vaccinations are the most cost effective way to protect our bodies against a variety of infectious microbes. The efficacy of most vaccines relies on protective antibody production and generation of memory B cells. These two key components develop mostly from B cells that participate in germinal center reactions. Recent efforts have highlighted the critical role of follicular helper CD4 T (Tfh) cells in the generation of germinal centers. Given that Bcl6 is a major transcription factor for Tfh differentiation, here we review recent developments in the understanding of signaling molecules that regulate Bcl6 expression in CD4 T cells, as a potential target for development of more efficacious vaccines.

Gambogic acid inhibits STAT3 phosphorylation through activation of protein tyrosine phosphatase SHP-1: potential role in proliferation and apoptosis

The transcription factor, STAT3, is associated with proliferation, survival, and metastasis of cancer cells. We investigated whether gambogic acid (GA), a xanthone derived from the resin of traditional Chinese medicine, Garcinia hanburyi (mangosteen), can regulate the STAT3 pathway, leading to suppression of growth and sensitization of cancer cells. We found that GA induced apoptosis in human multiple myeloma cells that correlated with the inhibition of both constitutive and inducible STAT3 activation. STAT3 phosphorylation at both tyrosine residue 705 and serine residue 727 was inhibited by GA. STAT3 suppression was mediated through the inhibition of activation of the protein tyrosine kinases Janus-activated kinase 1 (JAK1) and JAK2. Treatment with the protein tyrosine phosphatase (PTP) inhibitor pervanadate reversed the GA-induced downregulation of STAT3, suggesting the involvement of a PTP. We also found that GA induced the expression of the PTP SHP-1. Deletion of the SHP-1 gene by siRNA suppressed the ability of GA to inhibit STAT3 activation and to induce apoptosis, suggesting the critical role of SHP-1 in its action. Moreover, GA downregulated the expression of STAT3-regulated antiapoptotic (Bcl-2, Bcl-xL, and Mcl-1), proliferative (cyclin D1), and angiogenic (VEGF) proteins, and this correlated with suppression of proliferation and induction of apoptosis. Overall, these results suggest that GA blocks STAT3 activation, leading to suppression of tumor cell proliferation and induction of apoptosis.

Stat3: linking inflammation to epithelial cancer - more than a "gut" feeling?

Inflammation is an important environmental factor that promotes tumourigenesis and the progression of established cancerous lesions, and recent studies have started to dissect the mechanisms linking the two pathologies. These inflammatory and infectious conditions trigger immune and stromal cell release of soluble mediators which facilitate survival and proliferation of tumour cells in a paracrine manner. In addition, (epi-)genetic mutations affecting oncogenes, tumour-suppressor genes, chromosomal rearrangements and amplifications trigger the release of inflammatory mediators within the tumour microenvironment to promote neoplastic growth in an autocrine manner. These two pathways converge in tumour cells and result in activation of the latent signal transducer and activator of transcription 3 (Stat3) which mediates a transcriptional response favouring survival, proliferation and angiogenesis. The abundance of cytokines that activate Stat3 within the tumour microenvironment, which comprises of members of the interleukin (IL) IL6, IL10 and IL17/23 families, underpins a signaling network that simultaneously promotes the growth of neoplastic epithelium, fuels inflammation and suppresses the host's anti-tumour immune response. Accordingly, aberrant and persistent Stat3 activation is a frequent observation in human cancers of epithelial origin and is often associated with poor outcome.

Here we summarize insights gained from mice harbouring mutations in components of the Stat3 signaling cascade and in particular of gp130, the shared receptor for the IL6 family of cytokines. We focus on the various feed-back and feed-forward loops in which Stat3 provides the signaling node in cells of the tumour and its microenvironment thereby functionally linking excessive inflammation to neoplastic growth. Although these observations are particularly pertinent to gastrointestinal tumours, we suggest that the tumour's addiction to persistent Stat3 activation is likely to also impact on other epithelial cell-derived cancers. These insights provide clues to the judicious interference of the gp130/Stat3 signaling cascade in therapeutically targeting cancer.

Cannabinoid 1 receptor and interleukin-6 receptor together induce integration of protein kinase and transcription factor signaling to trigger neurite outgrowth

Activation of the G(o/i)-coupled cannabinoid 1 receptor (CB1R) has been shown to induce neurite outgrowth in Neuro2A cells through activation of Src kinase and STAT3 transcription factor. Signaling by the interleukin 6 receptor (IL-6R) also activates STAT3 through Jak kinase. We studied if signals from the two pathways could be integrated in a synergistic manner to trigger neurite outgrowth in Neuro2A cells. At low concentrations, when agonist at either receptor by itself has no effect, we found that CB1R and IL-6R stimulation together induced synergistic neurite outgrowth. Signal integration requires activation of transcription factors by Src, Jak, and mitogen-activated protein kinases. Mitogen-activated protein kinase can be activated by both receptors and shows enhanced early activation in the presence of both ligands. CREB and STAT3 transcription factors are required for synergy and show enhanced DNA-binding activity when both receptors are activated. STAT3 plays a critical role in integration of the signals downstream of the two receptors. When both pathways are activated, STAT3 phosphorylation is sustained for 6 h. This prolonged activation of STAT3 requires deactivation of SHP2 phosphatase. Reduction of SHP2 levels by RNA interference results in greater synergy in neurite outgrowth. Simultaneous knockdown of both SHP2 and STAT3 blocks the synergistic triggering of neurite outgrowth, indicating that STAT3 is downstream of SHP2. CB1R and IL-6R co-stimulation enhanced the differentiation of rat cortical neuron primary cultures. These results provide a mechanism where multiple protein kinases and transcription factors interact to integrate signals from G protein-coupled and cytokine receptor to evoke neurite outgrowth in Neuro2A cells.

The protein-tyrosine phosphatase, SRC homology-2 domain containing protein tyrosine phosphatase-2, is a crucial mediator of exogenous insulin-like growth factor signaling to human trophoblast

Adequate fetal growth depends on placental transfer of nutrients and gases from the mother; thus, as pregnancy progresses, the placenta must grow to meet the increasing demands of the developing fetus. IGFs control proliferation, differentiation, and survival of trophoblast in first-trimester placenta via intracellular tyrosine kinase signaling cascades, the activation of which is also regulated by tyrosine phosphatases. The protein-tyrosine phosphatase, Src homology-2 domain containing protein tyrosine phosphatase (SHP)-2, is crucial for mouse placental development and is known to mediate IGF actions in other systems. In this study we examined the role of SHP-2 in regulating IGF-mediated proliferation in human trophoblast. Immunohistochemical analysis demonstrated that SHP-2 is expressed strongly in cytotrophoblast and only weakly in syncytium. After small interfering RNA-mediated knockdown of SHP-2 in BeWo choriocarcinoma cells and human first-trimester placental explants, IGF-induced trophoblast proliferation, examined using immunohistochemical analysis of Ki67 and 5-bromo-2'-deoxyuridine incorporation, was significantly reduced (P < 0.05). Kinase activation assays suggested that SHP-2 interacts with the MAPK pathway to mediate these effects. Markers of trophoblast differentiation were elevated after SHP-2 knockdown. This study demonstrates a role for tyrosine phosphatases in human trophoblast and establishes SHP-2 as a component of the IGF signaling pathway that is required for normal placental growth.

Signal transducer and activator of transcription-3, inflammation, and cancer: how intimate is the relationship?

Signal transducer and activator of transcription-3 (STAT-3) is one of six members of a family of transcription factors. It was discovered almost 15 years ago as an acute-phase response factor. This factor has now been associated with inflammation, cellular transformation, survival, proliferation, invasion, angiogenesis, and metastasis of cancer. Various types of carcinogens, radiation, viruses, growth factors, oncogenes, and inflammatory cytokines have been found to activate STAT-3. STAT-3 is constitutively active in most tumor cells but not in normal cells. Phosphorylation of STAT-3 at tyrosine 705 leads to its dimerization, nuclear translocation, DNA binding, and gene transcription. The phosphorylation of STAT-3 at serine 727 may regulate its activity negatively or positively. STAT-3 regulates the expression of genes that mediate survival (survivin, bcl-xl, mcl-1, cellular FLICE-like inhibitory protein), proliferation (c-fos, c-myc, cyclin D1), invasion (matrix metalloproteinase-2), and angiogenesis (vascular endothelial growth factor). STAT-3 activation has also been associated with both chemoresistance and radioresistance. STAT-3 mediates these effects through its collaboration with various other transcription factors, including nuclear factor-kappaB, hypoxia-inducible factor-1, and peroxisome proliferator activated receptor-gamma. Because of its critical role in tumorigenesis, inhibitors of this factor's activation are being sought for both prevention and therapy of cancer. This has led to identification of small peptides, oligonucleotides, and small molecules as potential STAT-3 inhibitors. Several of these small molecules are chemopreventive agents derived from plants. This review discusses the intimate relationship between STAT-3, inflammation, and cancer in more detail.

Signal transducer and activator of transcription-3, inflammation, and cancer: how intimate is the relationship?

Signal transducer and activator of transcription-3 (STAT-3) is one of six members of a family of transcription factors. It was discovered almost 15 years ago as an acute-phase response factor. This factor has now been associated with inflammation, cellular transformation, survival, proliferation, invasion, angiogenesis, and metastasis of cancer. Various types of carcinogens, radiation, viruses, growth factors, oncogenes, and inflammatory cytokines have been found to activate STAT-3. STAT-3 is constitutively active in most tumor cells but not in normal cells. Phosphorylation of STAT-3 at tyrosine 705 leads to its dimerization, nuclear translocation, DNA binding, and gene transcription. The phosphorylation of STAT-3 at serine 727 may regulate its activity negatively or positively. STAT-3 regulates the expression of genes that mediate survival (survivin, bcl-xl, mcl-1, cellular FLICE-like inhibitory protein), proliferation (c-fos, c-myc, cyclin D1), invasion (matrix metalloproteinase-2), and angiogenesis (vascular endothelial growth factor). STAT-3 activation has also been associated with both chemoresistance and radioresistance. STAT-3 mediates these effects through its collaboration with various other transcription factors, including nuclear factor-kappaB, hypoxia-inducible factor-1, and peroxisome proliferator activated receptor-gamma. Because of its critical role in tumorigenesis, inhibitors of this factor's activation are being sought for both prevention and therapy of cancer. This has led to identification of small peptides, oligonucleotides, and small molecules as potential STAT-3 inhibitors. Several of these small molecules are chemopreventive agents derived from plants. This review discusses the intimate relationship between STAT-3, inflammation, and cancer in more detail.

Signal transducer and activator of transcription-3, inflammation, and cancer: how intimate is the relationship?

Signal transducer and activator of transcription-3 (STAT-3) is one of six members of a family of transcription factors. It was discovered almost 15 years ago as an acute-phase response factor. This factor has now been associated with inflammation, cellular transformation, survival, proliferation, invasion, angiogenesis, and metastasis of cancer. Various types of carcinogens, radiation, viruses, growth factors, oncogenes, and inflammatory cytokines have been found to activate STAT-3. STAT-3 is constitutively active in most tumor cells but not in normal cells. Phosphorylation of STAT-3 at tyrosine 705 leads to its dimerization, nuclear translocation, DNA binding, and gene transcription. The phosphorylation of STAT-3 at serine 727 may regulate its activity negatively or positively. STAT-3 regulates the expression of genes that mediate survival (survivin, bcl-xl, mcl-1, cellular FLICE-like inhibitory protein), proliferation (c-fos, c-myc, cyclin D1), invasion (matrix metalloproteinase-2), and angiogenesis (vascular endothelial growth factor). STAT-3 activation has also been associated with both chemoresistance and radioresistance. STAT-3 mediates these effects through its collaboration with various other transcription factors, including nuclear factor-kappaB, hypoxia-inducible factor-1, and peroxisome proliferator activated receptor-gamma. Because of its critical role in tumorigenesis, inhibitors of this factor's activation are being sought for both prevention and therapy of cancer. This has led to identification of small peptides, oligonucleotides, and small molecules as potential STAT-3 inhibitors. Several of these small molecules are chemopreventive agents derived from plants. This review discusses the intimate relationship between STAT-3, inflammation, and cancer in more detail.

Proinflammatory stem cell signaling in cardiac ischemia

Cardiovascular disease remains a leading cause of mortality in developed nations, despite continued advancement in modern therapy. Progenitor and stem cell-based therapy is a novel treatment for cardiovascular disease, and modest benefits in cardiac recovery have been achieved in small clinical trials. This therapeutic modality remains challenged by limitations of low donor-cell survival rates, transient recovery of cardiac function, and the technical difficulty of applying directed cell therapy. Understanding the signaling mechanisms involved in the stem cell response to ischemia has revealed opportunities to modify directly aspects of these pathways to improve their cardioprotective abilities. This review highlights general considerations of stem cell therapy for cardiac disease, reviews the major proinflammatory signaling pathways of mesenchymal stem cells, and reviews ex vivo modifications of stem cells based on these pathways.

IL-27 activates human monocytes via STAT1 and suppresses IL-10 production but the inflammatory functions of IL-27 are abrogated by TLRs and p38

IL-27 is a member of the IL-12 family of cytokines that activates the Jak-STAT signaling pathway in a context-dependent manner and has pleiotropic effects on acquired immunity. IL-27 has the capacity to promote early stages of Th1 generation, but recent evidence has suggested a predominant suppressive effect on Th1, Th2, and Th17 differentiation. Although modest suppressive effects of IL-27 on myeloid lineage cells have been observed, there is limited knowledge about the role of IL-27 in the regulation of innate immunity. In this study we report that although in resting murine macrophages IL-27 had minimal if any effects, in resting human monocytes IL-27 had profound proinflammatory functions. IL-27 activated a STAT1-dominant pattern of signaling in human monocytes with the consequent activation of STAT1-dependent inflammatory target genes. IL-27 primed monocytes for augmented responses to TLR stimulation in a STAT1-dependent manner, altered IL-10 signaling, and attenuated IL-10-induced gene expression. Strikingly, IL-27 strongly suppressed TLR-induced IL-10 production in human monocytes. However, the proinflammatory effects of IL-27 on human monocytes were rapidly abrogated by LPS via a p38-mediated mechanism that inhibited IL-27 signaling. Our findings identify a predominantly proinflammatory function for IL-27 in human monocytes and suggest a mechanism by which the activating effects of IL-27 on innate immunity are attenuated as an immune response proceeds and IL-27 transitions to predominantly suppressive effects on acquired immunity.

Interleukin-6 induces proliferation in adult spinal cord-derived neural progenitors via the JAK2/STAT3 pathway with EGF-induced MAPK phosphorylation

INTRODUCTION

In a previous study, we observed cell proliferation 3 days after spinal cord injury, and levels of interleukin-6 (IL-6) and epidermal growth factor (EGF) had significantly increased in the region of the injury.

OBJECTIVES

The purpose of the new study described here was to evaluate the roles of IL-6 and EGF after traumatic damage to the spinal cord having isolated neural progenitor cells (NPC) from adult mice.

METHODS AND RESULTS

Evidence provided by the trypan blue dye exclusion assay, 5-bromodeoxyuridine immunoreactivity and Western blot analysis indicated that IL-6 and EGF induced proliferation of these spinal cord-derived NPCs via phosphorylation of Janus-activated kinase 2 (JAK2)/signal transducer and activator of transcription 3 (STAT3) and mitogen-activated protein kinases (MAPK), respectively. Combined treatment with IL-6 and EGF accelerated proliferation of cells synergistically and phosphorylation of STAT3 and extracellular signal-regulated kinase 1/2 (Erk1/2). Furthermore, AG490 and AG1478, JAK2 inhibitor and EGFR inhibitor, respectively, prevented the IL-6- and EGF-induced proliferation of the cells. Interestingly, IL-6-activated MAPKs but EGF did not influence JAK2/STAT3 activation; AG490 specifically inhibited IL-6-induced Erk1/2 phosphorylation without affecting IL-6-induced phosphorylation of Raf and MEK1/2. These results indicate that IL-6 is directly involved in Erk1/2 activation via JAK2 and that Erk1/2 provides a signal bridge between the IL-6-induced JAK2/STAT3 pathway and EGF-induced MAPK pathway.

CONCLUSIONS

Our study is the first demonstration of IL-6- and EGF-stimulated proliferation of spinal cord progenitor cells via JAK2/STAT3 and MAPK signalling pathways. These pathways play key roles in repopulation and regeneration of spinal cord tissue after injury. It may represent novel therapeutic targets for pharmacological intervention in central nervous system disease, including spinal cord injury.

Augmented gp130-mediated cytokine signalling accompanies human gastric cancer progression

H. pylori infection accounts for most cases of gastric cancer, but the initiating events remain unclear. The principal H. pylori pathogenicity-associated CagA protein disrupts intracellular SHP-2 signalling pathways including those used by the IL-6 family cytokines, IL-6 and IL-11. Imbalanced IL-6 family cytokine signalling in the gp130(757FF) mouse model of gastric cancer arising from hyperactivation of oncogenic STAT3 after altered SHP-2 : ERK1/2 signalling produces dysplastic antral tumours preceded by gastritis and metaplasia. In a cohort of patient gastric biopsies with known H. pylori and CagA status, we investigated whether (i) STAT3 and ERK1/2 activation is altered in H. pylori-dependent gastritis; (ii) these profiles are more pronounced in CagA+ H. pylori infection; and (iii) the expression of pro-inflammatory cytokines that activate STAT3 and ERK 1/2 pathways is associated with progression to gastric cancer. IL-6, IL-11, and activated STAT3 and ERK1/2 were quantified in antral biopsies from gastritic stomach, metaplastic tissue, and resected gastric cancer tissues. We observed significantly increased STAT3 and ERK1/2 activation (p = 0.001) in H. pylori-dependent gastritis, which was further enhanced in the presence of CagA+ H. pylori strains. Of known gastric ligands that drive STAT3 activation, IL-6 expression was increased after H. pylori infection and both IL-6 and IL-11 were strongly up-regulated in the gastric cancer biopsies. This suggests a mechanism by which IL-11 drives STAT3 activation and proliferation during gastric cancer progression. We addressed this using an in vitro approach, demonstrating that recombinant human IL-11 activates STAT3 and concomitantly increases proliferation of MKN28 gastric epithelial cells. In summary, we show increased STAT3 and ERK1/2 activation in H. pylori-dependent gastritis that is likely driven in an IL-6-dependent fashion. IL-11 expression is associated with adenocarcinoma development, but not gastritic lesions, and we identify a novel mechanism for IL-11 as a potent inducer of proliferation in the human gastric cancer setting.

Interleukin-31 and oncostatin-M mediate distinct signaling reactions and response patterns in lung epithelial cells

Lung epithelial cells are primary targets of oncostatin M (OSM) and, to a lower degree, of interleukin (IL)-6 and IL-31, all members of the IL-6 cytokine family. The OSM receptor (OSMR) signals through activation of STAT and mitogen-activated protein kinase pathways to induce genes encoding differentiated cell functions, reduce cell-cell interaction, and suppress cell proliferation. IL-31 functions through the heteromeric IL-31 receptor, which shares with OSMR the OSMRbeta subunit, but does not engage gp130, the common subunit of all other IL-6 cytokine receptors. Because the response of epithelial cells to IL-31 is unknown, the action of IL-31 was characterized in the human alveolar epithelial cell line A549 in which the expression of the ligand-binding IL-31Ralpha subunit was increased. IL-31 initiated signaling that differed from other IL-6 cytokines by the particularly strong recruitment of the STAT3, ERK, JNK, and Akt pathways. IL-31 was highly effective in suppressing proliferation by altering expression of cell cycle proteins, including up-regulation of p27(Kip1) and down-regulation of cyclin B1, CDC2, CDK6, MCM4, and retinoblastoma. A single STAT3 recruitment site (Tyr-721) in the cytoplasmic domain of IL-31Ralpha exerts a dominant function in the entire receptor complex and is critical for gene induction, morphological changes, and growth inhibition. The data suggest that inflammatory and immune reactions involving activated T-cells regulate functions of epithelial cells by IL-6 cytokines through receptor-defined signaling reactions.

Concerted functions of Gab1 and Shp2 in liver regeneration and hepatoprotection

Liver regeneration is a rapid and concerted response to injury, in which growth factor-generated intracellular signals result in activation of transcription factors, DNA synthesis, and hepatocyte proliferation. However, the link between cytoplasmic signals resulting in proliferative response to liver injury remains to be elucidated. We show here that association of Gab1 adaptor protein and Shp2 tyrosine phosphatase is a critical event at the early phase of liver regeneration. Partial hepatectomy (PH) rapidly and transiently induced assembly of a complex comprising Shp2 and tyrosine-phosphorylated Gab1 in wild-type hepatocytes. Consistently, liver-specific Shp2 knockout (LSKO) and liver-specific Gab1 knockout (LGKO) mice displayed very similar phenotypes of defective liver regeneration triggered by PH, including blunted extracellular signal-regulated kinase 1/2 (Erk1/2) activation, decreased expression of immediate-early genes, and reduced levels of cyclins A, E, and B1, as well as suppression of hepatocyte proliferation. In contrast, the Akt and interleukin-6/Stat3 pathways were up-regulated posthepatectomy in LSKO and LGKO mice, accompanied by improved hepatoprotection. Collectively, this study establishes the physiological significance of the Gab1/Shp2 link in promoting mitogenic signaling through the Erk pathway in mammalian liver regeneration.

Sp3 is involved in the regulation of SOCS3 gene expression

Cytokine-induced expression of SOCS (suppressor of cytokine signalling) molecules is important for the negative regulatory control of STAT (signal transduction and activators of transcription)-dependent cytokine signalling, e.g. for the signal transduction of IL-6 (interleukin-6)-type cytokines through the JAK (Janus kinase)/STAT cascade. STAT activation itself represents an important step in the transcriptional activation of SOCS3 gene expression. However, downstream of the STAT-responsive element, the SOCS3 gene contains a GC-rich element in its 5'-upstream region. The aim of the present study was to investigate the implications of this GC-rich element in the transcriptional control of SOCS3 gene expression. In the present study, we show that mutation of this GC-rich element abolishes IL-6-dependent transcriptional activation of the SOCS3 promoter and that Sp3 (specificity protein 3), a ubiquitously expressed transcription factor, but not Sp1 binds to this GC-rich motif, suggesting that Sp3 is involved in the regulation of SOCS3 expression. The results suggest that Sp3 is important for IL-6-induced transcriptional activation of the SOCS3 (gene) promoter and acts as an enhancer of basal as well as induced transcriptional activity, resulting in enhanced SOCS3 mRNA and protein expression. Mutation of Lys-483, a potential target for Sp3 acetylation, inhibited Sp3-mediated enhancement of SOCS3 mRNA expression and SOCS3 promoter activation, indicating that the acetylation of this lysine residue of Sp3 is important for the enhancing effect of Sp3 on SOCS3 expression.

SOCS3 negatively regulates LIF signaling in neural precursor cells

Cytokines that signal through the LIFRbeta/gp130 receptor complex, including LIF and CNTF, promote the self-renewal of embryonic and adult neural precursor cells (NPCs). In non-CNS tissues, the protein suppressor of cytokine signaling-3 (SOCS3) negatively regulates signaling through gp130. Here, we analyze the role of SOCS3 in inhibiting LIF signaling in NPCs in vitro. SOCS3 is rapidly expressed by NPCs in response to LIF stimulation, with this expression largely dependent on recruitment of STAT proteins to the activated gp130 receptor. Proliferating NPC cultures can be generated from SOCS3 knockout (SOCS3KO/KO) embryos and display prolonged STAT3 phosphorylation and induction of the GFAP gene in response to LIF. In comparison with SOCS3 wild-type (SOCS3WT/WT) NPCs, SOCS3KO/KO cultures display enhanced self-renewal capacity. However, the clonal potential of SOCS3WT/WT but not SOCS3KO/KO NPCs is enhanced by exogenous LIF. Thus, SOCS3 acts as a negative regulator of LIF signaling in NPCs.

Mechanisms of SOCS3 phosphorylation upon interleukin-6 stimulation. Contributions of Src- and receptor-tyrosine kinases

The suppressors of cytokine signaling (SOCS) are negative feedback inhibitors of cytokine signal transduction. SOCS3 is a key negative regulator of interleuking-6 (IL-6) signal transduction. Furthermore, SOCS3 was shown to be phosphorylated upon treatment of cells with IL-2, and this has been reported to regulate its function and half-life. We set out to investigate whether SOCS3 phosphorylation may play a role in IL-6 signaling. Tyrosine-phosphorylated SOCS3 was detected upon treatment of mouse embryonic fibroblasts with IL-6. Interestingly, the observed SOCS3 phosphorylation does not require SOCS3 recruitment to phosphotyrosine (Tyr(P)) 759 of gp130, and the kinetics of SOCS3 phosphorylation do not match the activation kinetics of the Janus kinases. This suggests that other kinases may be involved in SOCS3 phosphorylation. Using Src and Janus kinase inhibitors as well as Src kinase-deficient mouse embryonic fibroblasts, we provide evidence that Src kinases, which we found to be constitutively active in these cells, are involved in the phosphorylation of IL-6-induced SOCS3. In addition, we found that receptor-tyrosine kinases such as platelet-derived growth factor receptor or epidermal growth factor receptor can very potently phosphorylate IL-6-induced SOCS3. Taken together, these results suggest that SOCS3 phosphorylation is not a JAK-mediated phenomenon but is dependent on the activity of other kinases such as Src kinases or receptor-tyrosine kinases, which can either be constitutively active or activated by an additional stimulus.

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.

Protein tyrosine phosphatases: their role in insulin action and potential as drug targets

Protein tyrosine phosphatases (PTPases) are the enzymes responsible for the selective dephosphorylation of tyrosine residues. PTPases function to regulate a wide array of biological responses mediated by growth factors and other stimuli by balancing the cellular level of phosphotyrosine in concert with their counterparts, protein tyrosine kinases. The important roles which PTPases play in regulating intracellular signalling and, ultimately, biological function along with the recent availability of information regarding their structural features has highlighted them as potential targets for pharmacological modulation. This is demonstrated by the increased level of activity directed towards the identification of novel small-molecule PTPase inhibitors. The rationale and potential utility of this drug discovery approach is discussed here, with particular emphasis on its application for the treatment of insulin resistance and Type 2 diabetes.

Noonan syndrome type I with PTPN11 3 bp deletion: Structure-function implications

Noonan syndrome was recently reported to be caused by mutations in the PTPN11 gene in 40% of the cases. This gene encodes the nonreceptor-type protein tyrosine phosphatase SHP-2 and has been shown to be self down-regulated with the concurrency of two SH2 domains. Insertion of a specific loop (D'EF) from N-terminal SH2 domain into the SHP-2 active-site is responsible for the reversible inhibition of the phosphatase activity. Here we report the first in frame trinucleotide deletion resulting in the removal of Aspartate 61 (D61del), a key residue of the N-terminal SH2 D'EF loop. Energetic-based structural analysis and electrostatic calculations carried out on the wild-type and mutant proteins predict lower stability of the D'EF loop for the D61del variant as compared to the wild type indicating better access to the active site and most likely an enzyme activated for longer extent. Similar computations were performed on the previously functionally characterized gain-of-function D61Y mutant and similar behaviors were observed. The simulation data for the D61del and D61Y mutants suggest that both variants could yield more catalytic cycles than the wild-type molecule in the same timespan because of the opening of the active site. It also supports the notion that D61 plays a major role for proper down-regulation of the protein tyrosine phosphatase activity of SHP-2.

Phosphorylation of Grb2-Associated Binder 2 on Serine 623 by ERK MAPK Regulates Its Association with the Phosphatase SHP-2 and Decreases STAT5 Activation

IL-2 stimulation of T lymphocytes induces the tyrosine phosphorylation and adaptor function of the insulin receptor substrate/Grb2-associated binder (Gab) family member, Gab2. In addition, Gab2 undergoes a marked decrease in its mobility in SDS-PAGE, characteristic of migration shifts induced by serine/threonine phosphorylations in many proteins. This migration shift was strongly diminished by treating cells with the MEK inhibitor U0126, indicating a possible role for ERK in Gab2 phosphorylation. Indeed, ERK phosphorylated Gab2 on a consensus phosphorylation site at serine 623, a residue located between tyrosine 614 and tyrosine 643 that are responsible for Gab2/Src homology 2 domain-containing tyrosine phosphatase (SHP)-2 interaction. We report that pretreatment of Kit 225 cells with U0126 increased Gab2/SHP-2 association and tyrosine phosphorylation of SHP-2 in response to IL-2, suggesting that ERK phosphorylation of serine 623 regulates the interaction between Gab2 and SHP-2, and consequently the activity of SHP-2. This hypothesis was confirmed by biochemical analysis of cells expressing Gab2 WT, Gab2 serine 623A or Gab2 tyrosine 614F, a mutant that cannot interact with SHP-2 in response to IL-2. Activation of the ERK pathway was indeed blocked by Gab2 tyrosine 614F and slightly increased by Gab2 serine 623A. In contrast, STAT5 activation was strongly enhanced by Gab2 tyrosine 614F, slightly reduced by Gab2 WT and strongly inhibited by Gab2 serine 623A. Analysis of the rate of proliferation of cells expressing these mutants of Gab2 demonstrated that tyrosine 614F mutation enhanced proliferation whereas serine 623A diminished it. These results demonstrate that ERK-mediated phosphorylation of Gab2 serine 623 is involved in fine tuning the proliferative response of T lymphocytes to IL-2.

The role of the inhibitors of interleukin-6 signal transduction SHP2 and SOCS3 for desensitization of interleukin-6 signalling

The immediate early response of cells treated with IL-6 (interleukin-6) is the activation of the signal transducer and activator of transcription (STAT)3. The Src homology domain 2 (SH2)-containing protein tyrosine phosphatase SHP2 and the feedback inhibitor SOCS3 (suppressor of cytokine signalling) are potent inhibitors of IL-6 signal transduction. Impaired function of SOCS3 or SHP2 leads to enhanced and prolonged IL-6 signalling. The inhibitory function of both proteins depends on their recruitment to the tyrosine motif 759 within glycoprotein gp130. In contrast to inactivation, desensitization of signal transduction is regarded as impaired responsiveness due to prestimulation. Usually, after activation the sensing receptor becomes inactivated by modifications such as phosphorylation, internalization or degradation. We designed an experimental approach which allows discrimination between desensitization and inactivation of IL-6 signal transduction. We observed that pre-stimulation with IL-6 renders cells less sensitive to further stimulation with IL-6. After several hours, the cells become sensitive again. We show that not only signal transduction through previously activated receptors is affected by desensitization but signalling through receptors which were not targeted by the first stimulation was also attenuated ( trans -desensitization). Interestingly, in contrast to inhibition, desensitization does not depend on the presence of functional SHP2. Furthermore, cells lacking SOCS3 show constitutive STAT3 activation which is not affected by pre-stimulation with IL-6. All these observations suggest that desensitization and inhibition of signalling are mechanistically distinct.

ERK1/2 contributes negative regulation to STAT3 activity in HSS-transfected HepG2 cells

Signal transducer and activator of transcription 3 (STAT3) is a recently characterized transcription factor which is essential to liver regeneration. We have previously reported that hepatic stimulator substance (HSS), a novel growth-promoting substance, phosphorylated the epidermal growth factor (EGF) receptors and activated downstream Ras-MAP kinase (extracellular signal-regulated kinases, ERK1/2) cascade. However, whether HSS signal is related to STAT3 pathway remains unclear. The present study is aiming to explore the regulatory effect of activation of ERK1/2 evoked by HSS on STAT3 phosphorylation and STAT3 signaling. Human hepatoma cell line HepG2 was stably transfected with HSS cDNA and HSS expression was measured by Northern blot. The results showed that the transfection of HSS into HepG2 resulted in remarkable increase in cellular proliferation as compared with the non-transfected cells, and it was further proved that the cellular proliferation in the HSS-transfected cells was related to ERK1/2 activation. Treatment of the cells with 50 mM of PD98059, an ERK1/2 specific upstream inhibitor, resulted in ERK1/2 inactivation completely. Inhibition of ERK1/2 allowed the tyrosine of STAT3 to be phosphorylated in a dose-dependent manner to PD98059. Furthermore, transient transfection of STAT3 mutant (STAT3S727A) into HSS-bearing cells could remarkably reverse the inhibitory effect of ERK1/2 on STAT3 phosphorylation. Based upon these results, it is concluded that ERK1/2 negatively modulates STAT3 phosphorylation and this function is dependent on residual serine-727 (S727) of STAT3.

Imbalanced gp130-dependent signaling in macrophages alters macrophage colony-stimulating factor responsiveness via regulation of c-fms expression

The mechanisms by which interleukin-6 (IL-6) family cytokines, which utilize the common receptor signaling subunit gp130, influence monocyte/macrophage development remain unclear. Here we have utilized macrophages devoid of either gp130-dependent STAT1/3 (gp130(Delta STAT/Delta STAT)) or extracellular signal-regulated kinases 1 and 2 (ERK1/2) mitogen-activated protein (MAP) kinase (gp130(Y757F/Y757F)) activation to assess the individual contribution of each pathway to macrophage formation. While the inhibition by IL-6 of macrophage colony-stimulating factor (M-CSF)-induced colony formation observed in gp130(wt/wt) mice was abolished in gp130(Delta STAT/Delta STAT) mice, inhibition of macrophage colony formation was enhanced in gp130(Y757F/Y757F) mice. In gp130(Delta STAT/Delta STAT) bone marrow-derived macrophages (BMMs), both IL-6- and M-CSF-induced ERK1/2 tyrosine phosphorylation was enhanced. By contrast, tyrosine phosphorylation of ERK1/2 in response to M-CSF was reduced in gp130(Y757F/Y757F) BMMs, and the pattern of ERK1/2 activation in gp130 mutant BMMs correlated with their opposing responsiveness to M-CSF-induced proliferation. When compared to the level of expression in gp130(wt/wt) BMMs, c-fms expression was elevated in gp130(Delta STAT/Delta STAT) BMMs but reduced in gp130(Y757F/Y757F) BMMs. Finally, an ERK1/2 inhibitor suppressed M-CSF-induced BMM proliferation, and this result corresponded to a reduction in c-fms expression. Collectively, these results provide a functional and causal correlation between gp130-dependent ERK MAP kinase signaling and c-fms gene activation, a finding that provides a potential mechanism underlying the inhibition of M-CSF-dependent macrophage development by IL-6 family cytokines in mice.

IL-6 signal transduction and its physiological roles: the signal orchestration model

Interleukin (IL)-6 is a pleiotropic cytokine that not only affects the immune system, but also acts in other biological systems and many physiological events in various organs. In a target cell, IL-6 can simultaneously generate functionally distinct or sometimes contradictory signals through its receptor complex, IL-6Ralpha and gp130. One good illustration is derived from the in vitro observations that IL-6 promotes the growth arrest and differentiation of M1 cells through gp130-mediated STAT3 activation, whereas the Y759/SHP-2-mediated cascade by gp130 stimulation has growth-enhancing effects. The final physiological output can be thought of as a consequence of the orchestration of the diverse signaling pathways generated by a given ligand. This concept, the signal orchestration model, may explain how IL-6 can elicit proinflammatory or anti-inflammatory effects, depending on the in vivo environmental circumstances. Elucidation of the molecular mechanisms underlying this issue is a challenging subject for future research. Intriguingly, recent in vivo studies indicated that the SHP-2-binding site- and YXXQ-mediated pathways through gp130 are not mutually exclusive but affect each other: a mutation at the SHP-2-binding site prolongs STAT3 activation, and a loss of STAT activation by gp130 truncation leads to sustained SHP-2/ERK MAPK phosphorylation. Although IL-6/gp130 signaling is a promising target for drug discovery for many human diseases, the interdependence of each signaling pathway may be an obstacle to the development of a nonpeptide orally active small molecule to inhibit one of these IL-6 signaling cascades, because it would disturb the signal orchestration. In mice, a consequence of the imbalanced signals causes unexpected results such as gastrointestinal disorders, autoimmune diseases, and/or chronic inflammatory proliferative diseases. However, lessons learned from IL-6 KO mice indicate that IL-6 is not essential for vital biological processes, but a significant impact on disease progression in many experimental models for human disorders. Thus, IL-6/gp130 signaling will become a more attractive therapeutic target for human inflammatory diseases when a better understanding of IL-6 signaling, including the identification of the conductor for gp130 signal transduction, is achieved.

Principles of interleukin (IL)-6-type cytokine signalling and its regulation

The IL (interleukin)-6-type cytokines IL-6, IL-11, LIF (leukaemia inhibitory factor), OSM (oncostatin M), ciliary neurotrophic factor, cardiotrophin-1 and cardiotrophin-like cytokine are an important family of mediators involved in the regulation of the acute-phase response to injury and infection. Besides their functions in inflammation and the immune response, these cytokines play also a crucial role in haematopoiesis, liver and neuronal regeneration, embryonal development and fertility. Dysregulation of IL-6-type cytokine signalling contributes to the onset and maintenance of several diseases, such as rheumatoid arthritis, inflammatory bowel disease, osteoporosis, multiple sclerosis and various types of cancer (e.g. multiple myeloma and prostate cancer). IL-6-type cytokines exert their action via the signal transducers gp (glycoprotein) 130, LIF receptor and OSM receptor leading to the activation of the JAK/STAT (Janus kinase/signal transducer and activator of transcription) and MAPK (mitogen-activated protein kinase) cascades. This review focuses on recent progress in the understanding of the molecular mechanisms of IL-6-type cytokine signal transduction. Emphasis is put on the termination and modulation of the JAK/STAT signalling pathway mediated by tyrosine phosphatases, the SOCS (suppressor of cytokine signalling) feedback inhibitors and PIAS (protein inhibitor of activated STAT) proteins. Also the cross-talk between the JAK/STAT pathway with other signalling cascades is discussed.

The role of gp130/IL-6 cytokines in the development of pulmonary fibrosis: critical determinants of disease susceptibility and progression?

Cryptogenic fibrosing alveolitis (CFA), also known as idiopathic pulmonary fibrosis (IPF), is the end stage of a heterogeneous group of disorders in which the deposition of excessive amounts of collagen results in the loss of lung function and premature death. The molecular mechanisms underlying the disease are unknown. Accordingly, there is much debate as to whether pulmonary fibrosis is the end result of (1) a chronic inflammatory process or (2) a disturbance in normal epithelium-fibroblast cross talk, or both. In addition, it appears increasingly likely that there is a genetic component in the development of pulmonary fibrosis. The IL-6 cytokine family is a group of pleiotropic mediators produced by a variety of cells in response to a inflammatory stimuli. These cytokines are grouped together on the basis of weak structural homology, overlapping functions, and shared use of the transmembrane glycoprotein beta-subunit gp130 as part of their multimeric receptor complexes. Activation of these receptor complexes results in the recruitment and phosphorylation of specific transcription factors. In addition, membrane-proximal tyrosine residues act as docking sites for molecules involved in the activation of extracellular signal-related kinase (ERK). However, studies in genetically engineered mice that overexpress members of this family have shown that while overlapping biological activities exist, there are effects specific to individual cytokines. Data from both human and animal studies are now emerging to suggest that members of this cytokine family play an important role in the pathogenesis of fibroproliferative diseases and thus represent a novel group of cytokines implicated in pulmonary fibrosis. Importantly, manipulation of signaling pathways activated by these cytokines may suppress fibrosis but leave innate cellular mechanisms necessary for host defense largely untouched. This may provide guides for the development of novel pharmacological treatment for fibroproliferative diseases.

Principles of interleukin (IL)-6-type cytokine signalling and its regulation

The IL (interleukin)-6-type cytokines IL-6, IL-11, LIF (leukaemia inhibitory factor), OSM (oncostatin M), ciliary neurotrophic factor, cardiotrophin-1 and cardiotrophin-like cytokine are an important family of mediators involved in the regulation of the acute-phase response to injury and infection. Besides their functions in inflammation and the immune response, these cytokines play also a crucial role in haematopoiesis, liver and neuronal regeneration, embryonal development and fertility. Dysregulation of IL-6-type cytokine signalling contributes to the onset and maintenance of several diseases, such as rheumatoid arthritis, inflammatory bowel disease, osteoporosis, multiple sclerosis and various types of cancer (e.g. multiple myeloma and prostate cancer). IL-6-type cytokines exert their action via the signal transducers gp (glycoprotein) 130, LIF receptor and OSM receptor leading to the activation of the JAK/STAT (Janus kinase/signal transducer and activator of transcription) and MAPK (mitogen-activated protein kinase) cascades. This review focuses on recent progress in the understanding of the molecular mechanisms of IL-6-type cytokine signal transduction. Emphasis is put on the termination and modulation of the JAK/STAT signalling pathway mediated by tyrosine phosphatases, the SOCS (suppressor of cytokine signalling) feedback inhibitors and PIAS (protein inhibitor of activated STAT) proteins. Also the cross-talk between the JAK/STAT pathway with other signalling cascades is discussed.

Desensitization of signaling by oncostatin M in human vascular cells involves cytoplasmic Tyr residue 759 in gp130 but is not mediated by either Src homology 2 domain-containing tyrosine phosphatase 2 or suppressor of cytokine signaling 3

Oncostatin M (OnM) signals through cell surface receptors, which utilize the gp130 subunit. In cultured human umbilical vein endothelial cells (HUVEC), OnM transiently elevates mRNA encoding for suppressor of cytokine signaling-3 (SOCS-3). By 1 h of OnM treatment, HUVEC become refractory to the restimulation by OnM, measured as failure to reinduce SOCS-3 mRNA. OnM-induced desensitization also prevents responses to other gp130-signaling cytokines (e.g. leukemia inhibitory factor and interleukin 11). OnM treatment does not affect gp130 expression levels and desensitizes signaling mediated by a transduced chimeric receptor containing extracellular domains of platelet-derived growth factor receptor-beta (PDGFRbeta) and the cytoplasmic region of gp130. Interestingly, a chimeric PDGFRbeta-gp130 mutant receptor, in which intracellular Tyr residue 759 of gp130 is replaced by a Phe residue, mediates prolonged signaling and is not cross-desensitized by OnM. Phospho-Tyr759 is the binding site for both SOCS-3 and for Src homology domain 2-containing tyrosine phosphatase 2 (SHP-2). In human aortic smooth muscle cells, neither prevention of SOCS-3 protein induction, using STAT3 or SOCS-3 antisense, nor prevention of SHP-2 expression, also with antisense, ablates desensitization. These data suggest that desensitization of vascular cells to OnM is mediated in trans and involves Tyr residue 759 in gp130 but is not mediated by either SOCS-3 or SHP-2, the only two proteins currently known to bind to gp130 at this site.

TNF-alpha induces tyrosine phosphorylation and recruitment of the Src homology protein-tyrosine phosphatase 2 to the gp130 signal-transducing subunit of the IL-6 receptor complex

Recently, it has been demonstrated that TNF-alpha and LPS induce the expression of suppressor of cytokine signaling 3 (SOCS3) and inhibit IL-6-induced STAT3 activation in macrophages. Inhibitor studies suggested that both induction of SOCS3 and inhibition of IL-6-induced STAT3 activation depend on the activation of p38 mitogen-activated protein kinase. Since recruitment of the tyrosine phosphatase Src homology protein tyrosine phosphatase 2 (SHP2) to the signal-transducing receptor subunit gp130 attenuates IL-6-mediated STAT-activation, we were interested in whether TNF-alpha also induces the association of SHP2 to the gp130 receptor subunit. In this study we demonstrate that stimulation of macrophages and fibroblast cell lines with TNF-alpha causes the recruitment of SHP2 to the gp130 signal-transducing subunit and leads to tyrosine phosphorylation of SHP2 and gp130. In this context the cytoplasmic SHP2/SOCS3 recruitment site of gp130 tyrosine 759 is shown to be important for the inhibitory effects of TNF-alpha, since mutation of this residue completely restores IL-6-stimulated activation of STAT3 and, consequently, of a STAT3-dependent promoter. In this respect murine fibroblasts lacking exon 3 of SHP2 are not sensitive to TNF-alpha, indicating that functional SHP2 and its recruitment to gp130 are key events in inhibition of IL-6-dependent STAT activation by TNF-alpha. Furthermore, activation of p38 mitogen-activated protein kinase is shown to be essential for the inhibitory effect of TNF-alpha on IL-6 signaling and TNF-alpha-dependent recruitment of SHP2 to gp130.

Negative regulation of gp130 signalling mediated through tyrosine-757 is not dependent on the recruitment of SHP2

Cytokines of the interleukin-6 family utilize the shared cytokine receptor gp130 in the formation of active signalling complexes. Tyrosine-757 (Y757) on this receptor is critical for negative regulation of gp130-mediated signalling. Two signalling regulators, suppressor of cytokine signalling 3 (SOCS3) and Src homology 2 domain-containing tyrosine phosphatase-2 (SHP2), are recruited to Y757 following receptor activation; however, the relative contribution made by each of these in down-regulating gp130 signalling is not known. In the present study, we show the design of a mutant gp130 receptor that can recruit SHP2, but not SOCS3. This receptor maintains the critical Y757 residue, but contains mutations in other surrounding residues which are also important for interactions with the Src homology 2 domains of SOCS3 and SHP2. Cells transfected with a chimaeric receptor containing the SHP2-selective gp130 intracellular domain showed an enhanced response to cytokine stimulation, which was similar to that shown by a chimaeric gp130 receptor mutant carrying a Y757F point mutation that failed to recruit either SOCS3 or SHP2. These results demonstrate that the recruitment of SHP2 alone is not sufficient for Y757-dependent negative regulation of gp130 signalling and that this activity must therefore be dependent on SOCS3.