Updating interleukin-6 classic- and trans-signaling

Essential Amino Acids-Rich Diet Decreased Adipose Tissue Storage in Adult Mice: A Preliminary Histopathological Study

Background: Excess body adipose tissue accumulation is a common and growing health problem caused by an unbalanced diet and/or junk food. Although the effects of dietary fat and glucose on lipid metabolism regulation are well known, those of essential amino acids (EAAs) have been poorly investigated. Our aim was to study the influence of a special diet containing all EAAs on retroperitoneal white adipose tissue (rpWAT) and interscapular brown adipose tissue (BAT) of mice. Methods: Two groups of male Balb/C mice were used. The first was fed with a standard diet. The second was fed with an EAAs-rich diet (EAARD). After 3 weeks, rpWAT and BAT were removed and prepared for subsequent immunohistochemical analysis. Results: EAARD, although consumed significantly less, moderately reduced body weight and BAT, but caused a massive reduction in rpWAT. Conversely, the triceps muscle increased in mass. In rpWAT, the size of adipocytes was very small, with increases in leptin, adiponectin and IL-6 immunostaining. In BAT, there was a reduction in lipid droplet size and a simultaneous increase in UCP-1 and SIRT-3. Conclusions: A diet containing a balanced mixture of free EAA may modulate body adiposity in mice, promoting increased thermogenesis.

Effects of the Interleukin-6 Receptor Blocker Sarilumab on Metabolic Activity and Differentiation Capacity of Primary Human Osteoblasts

Interleukin (IL-) 6 is a key factor in the inflammatory processes of rheumatoid arthritis. Several biologic agents target the IL-6 signaling pathway, including sarilumab, a monoclonal antibody that blocks the IL-6 receptor and inhibits IL-6-mediated cis- and trans-signaling. A careful analysis of the IL-6 signaling blockade should consider not only inflammatory processes but also the regenerative functions of IL-6. The purpose of this study was to investigate whether inhibition of the IL-6 receptors affects differentiation of human primary osteoblasts (hOB). The effects of sarilumab on viability and the differentiation capacity in unstimulated osteoblasts as well as after stimulation with various IL-6 and sIL6-R concentrations were determined. Sarilumab treatment alone did not affect the differentiation or induction of inflammatory processes in hOB. However, the significant induction of alkaline phosphatase activity which was observed after exogenous IL-6/sIL-6R costimulation at the highest concentrations was reduced back to baseline levels by the addition of sarilumab. The IL-6 receptor blockade also decreased gene expression of mediators required for osteogenesis and bone matrix maintenance. Our results demonstrate that concomitant administration of the IL-6 receptor blocker sarilumab can inhibit IL-6/sIL-6R-induced osteogenic differentiation.

Neuroinflammatory Triangle Presenting Novel Pharmacological Targets for Ischemic Brain Injury

Ischemic stroke is one of the leading causes of morbidity and mortality globally. Hundreds of clinical trials have proven ineffective in bringing forth a definitive and effective treatment for ischemic stroke, except a myopic class of thrombolytic drugs. That, too, has little to do with treating long-term post-stroke disabilities. These studies proposed diverse options to treat stroke, ranging from neurotropic interpolation to venting antioxidant activity, from blocking specific receptors to obstructing functional capacity of ion channels, and more recently the utilization of neuroprotective substances. However, state of the art knowledge suggests that more pragmatic focus in finding effective therapeutic remedy for stroke might be targeting intricate intracellular signaling pathways of the 'neuroinflammatory triangle': ROS burst, inflammatory cytokines, and BBB disruption. Experimental evidence reviewed here supports the notion that allowing neuroprotective mechanisms to advance, while limiting neuroinflammatory cascades, will help confine post-stroke damage and disabilities.

The role of IL-6 and IL-6 blockade in COVID-19

INTRODUCTION

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) induces a dysregulated hyperinflammatory response.

AREAS COVERED

Authors review evidence on IL-6 and IL-6 blockade in coronavirus disease 2019 (COVID-19) and discuss the pathophysiological and prognostic roles of this cytokine and the clinical impact of pharmacological blockade of IL-6 . The material includes original articles and reviews published from March 2020 to March 2021 and searched on PubMed, medRxiv, and bioRxiv.

EXPERT OPINION

IL-6 is one of the most prominent pro-inflammatory cytokines. Increased levels are recorded in COVID-19 patients, especially those with severe-to-critical disease. Evidence is accumulating on the relevance of IL-6 as a prognostic marker in COVID-19. Since IL-6 is a druggable target for several inflammatory diseases, pharmacological blockers of the IL-6 signaling pathway were repurposed to blunt the abnormal SARS-CoV-2-induced cytokine release. Data are limited to few randomized controlled trials that reported encouraging, though not conclusive, results, indicating the usefulness of IL-6 blockade early in the course of the disease in patients with hyperinflammation and no or limited organ damage. Further research is warranted to explore the role of IL-6 in different COVID-19 phenotypes and identify subgroups of patients who may mostly benefit from IL-6 pathway inhibition.

Local action of cytokines and immune cells in communication between the conceptus and uterus during the critical period of early embryo development, attachment and implantation - Implications for embryo survival in cattle: A review

Early embryo development, implantation and pregnancy involve a complex dialogue between the embryo and mother. In cattle this dialogue starts as early as days 3-4 when the embryo is still in the oviduct, and it continues to implantation. Immunological processes involving cytokines, mast cells and macrophages form an important part of this dialogue. Amongst the cytokines, interleukin-6 (Il-6) and leukemia inhibitory factor (LIF) are secreted by both the embryo and uterine endometrium and form part of an ongoing and reciprocating dialogue. Mast cells and macrophages populate the uterine endometrium during embryo development and are involved in achieving the correct balance between inflammatory and anti-inflammatory reactions at the uterus that are associated with embryo attachment and implantation. Embryo loss is the major cause of reproductive wastage in cattle, and livestock generally. A deeper understanding of immunological processes during early embryo development will help to achieve the next step change in the efficiency of natural and assisted breeding.

TH17 cells require ongoing classic IL-6 receptor signaling to retain transcriptional and functional identity

Acting in concert with TGF-β, interleukin-6 (IL-6) signaling induces T helper 17 (T_H17) cell development by programming T_H17-related genes via signal transducers and activators of transcription 3 (STAT3). A role for IL-6 signaling beyond the inductive phase of T_H17 cell development has not been defined because IL-23 signaling downstream of T_H17 cell induction also activates STAT3 and is thought responsible for T_H17 cell maintenance. Here, we find that IL-6 signaling is required for both induction and maintenance of mouse T_H17 cells; IL-6Rα-deficient T_H17 cells rapidly lost their T_H17 phenotype and did not cause disease in two models of colitis. Cotransfer of wild-type T_H17 cells with IL-6Rα-deficient T_H17 cells induced colitis but was unable to rescue phenotype loss of the latter. High IL-6 expression in the colon promoted classic, or cis, rather than transreceptor signaling that was required for maintenance of T_H17 cells. Thus, ongoing classic IL-6 signaling underpins the T_H17 program and is required for T_H17 cell maintenance and function.

Bioinformatics-Based Discovery of CKLF-Like MARVEL Transmembrane Member 5 as a Novel Biomarker for Breast Cancer

Chemokine-like factor (CKLF)-like MARVEL transmembrane members (CMTMs) represent a novel protein family linking the chemokine and transmembrane-4 superfamily families, which potentially play several roles in diverse physiological and pathological processes. The detailed functions and underlying molecular mechanisms of CMTMs remain elusive in breast cancer. Herein, we performed a comprehensive bioinformatic analysis to investigate the prognostic effect, potential functions, and biomolecular regulatory network of CMTMs in breast cancer. The mRNA expression level of CMTM5, in particular, was significantly downregulated in breast cancer; moreover, high mRNA expression level of CMTM5 was significantly associated with better relapse-free survival. DNA promoter hypermethylation of CMTM5 was negatively correlated with its mRNA expression levels. Furthermore, CMTM5 strongly associated with pathway in MARVEL domains, chemotaxis, cytokines, transmembrane structures, and integral component of membrane. For example, genes related to MARVEL domains, transmembrane structures, and chemokines were significantly enriched. Our findings indicate that CMTM5 can be used as a prognostic biomarker and potential therapeutic target for breast cancer.

Co-Expression of VEGF and IL-6 Family Cytokines is Associated with Decreased Survival in HER2 Negative Breast Cancer Patients: Subtype-Specific IL-6 Family Cytokine-Mediated VEGF Secretion

Breast cancer cell-response to inflammatory cytokines such as interleukin-6 (IL-6) and oncostatin M (OSM) may affect the course of clinical disease in a cancer subtype-dependent manner. Furthermore, vascular endothelial growth factor A (VEGF) secretion induced by IL-6 and OSM may also be subtype-dependent. Utilizing datasets from Oncomine, we show that poor survival of invasive ductal carcinoma (IDC) breast cancer patients is correlated with both high VEGF expression and high cytokine or cytokine receptor expression in tumors. Importantly, epidermal growth factor receptor-negative (HER2-), but not HER2-positive (HER2+), patient survival is significantly lower with high tumor co-expression of VEGF and OSM, OSMRβ, IL-6, or IL-6Rα compared to low co-expression. Furthermore, assessment of HER2- breast cancer cells in vitro identified unique signaling differences regulating cytokine-induced VEGF secretion. The levels of VEGF secretion were analyzed by ELISA with siRNAs for hypoxia inducible factor 1 α (HIF1α) and signal transducer and activator of transcription 3 (STAT3). Specifically, we found that estrogen receptor-negative (ER-) MDA-MB-231 cells respond only to OSM through STAT3 signaling, while ER+ T47D cells respond to both OSM and IL-6, though to IL-6 to a lesser extent. Additionally, in the ER+ T47D cells, OSM signals through both STAT3 and HIF1α. These results highlight that the survival of breast cancer patients with high co-expression of VEGF and IL-6 family cytokines is dependent on breast cancer subtype. Thus, the heterogeneity of human breast cancer in relation to IL-6 family cytokines and VEGF may have important implications in clinical treatment options, disease progression, and ultimately patient prognosis.

Reverse Engineering the Febrile System

Fever, the elevation of core body temperature by behavioral or physiological means, is one of the most salient aspects of human sickness, yet there is debate regarding its functional role. In this paper, we demonstrate that the febrile system is an evolved adaptation shaped by natural selection to coordinate the immune system to fight pathogens. First, we show that previous arguments in favor of fever being an adaptation are epistemologically inadequate, and we describe how an adaptationist strategy addresses this issue more effectively. Second, we argue that the mechanisms producing fever provide clear indications of adaptation. Third, we demonstrate that there are many beneficial immune system responses activated during fever and that these responses are not mere byproducts of heat on chemical reactions. Rather, we show that natural selection appears to have modified several immune system effects to be coordinated by fever. Fourth, we argue that there are some adaptations that coordinate the febrile system with other important fitness components, particularly growth and reproduction. Finally, we discuss evidence that the febrile system may also have evolved an antitumor function, providing suggestions for future research into this area. This research informs the debate on the functional value of fever and antipyretic use.

IL-6 signalling pathways and the development of type 2 diabetes

Interleukin 6 (IL-6), a multifunctional cytokine, has been implicated in the pathophysiology of type 2 diabetes (T2D). The elevated circulating level of IL-6 is an independent predictor of T2D and is considered to be involved in the development of inflammation, insulin resistance and β-cell dysfunction. On the other hand, an increasing number of evidence suggests that IL-6 has an anti-inflammatory role and improves glucose metabolism. The complex signal transduction mechanism of IL-6 may help explain the pleiotropic nature of the cytokine. IL-6 acts via two distinct signalling pathways called classic signalling and trans-signalling. While both signalling modes lead to activation of the same receptor subunit, their final biological effects are completely different. The aim of this review is to summarize our current knowledge about the role of IL-6 in the development of T2D. We will also discuss the importance of specific blockade of IL-6 trans-signalling rather than inhibiting both signalling pathways as a therapeutic strategy for the treatment of T2D and its associated macrovascular complications.

Butyltin compounds alter secretion of interleukin 6 from human immune cells

Butyltins (BTs), tributyltin (TBT) and dibutyltin (DBT) are organotin compounds that have been used in a variety of industrial applications; as a result, these compounds have been found in human blood. Interleukin (IL)-6 is a proinflammatory mediator that is produced by T lymphocytes and monocytes. It is responsible for immune response regulation as well as tissue repair and cellular growth. Both BTs decrease the ability of human natural killer cells to destroy tumor cells and alter the secretion of proinflammatory cytokines tumor necrosis factor alpha, interferon gamma and IL-1 beta (β) from human lymphocytes ex vivo. Here, we show that BTs alter the secretion of IL-6 from increasingly reconstituted preparations of human immune cells. IL-6 secretion was examined after 24 hour, 48 hour or 6 day exposures to TBT and DBT in highly enriched human natural killer cells, monocyte-depleted peripheral blood mononuclear cells (PBMCs), PBMCs, granulocytes and a preparation combining both PBMCs and granulocytes (PBMCs + granulocytes). The results indicated that both BTs altered IL-6 secretion from all cell preparations. Significant decreases of IL-6 secretion were seen at the highest concentration of TBT (200 nm) and DBT (5-2.5 μm) while the lower concentrations of DBT (0.05 and 0.1 μm) caused elevation of IL-6 secretion. The data indicate that BT-induced alterations of IL-6 secretion from immune cells may be a significant consequence of BT exposures that may potentially affect immune competence.

Interleukin-6 Signaling Pathway and Its Role in Kidney Disease: An Update

Interleukin-6 (IL-6) is a pleiotropic cytokine that not only regulates the immune and inflammatory response but also affects hematopoiesis, metabolism, and organ development. IL-6 can simultaneously elicit distinct or even contradictory physiopathological processes, which is likely discriminated by the cascades of signaling pathway, termed classic and trans-signaling. Besides playing several important physiological roles, dysregulated IL-6 has been demonstrated to underlie a number of autoimmune and inflammatory diseases, metabolic abnormalities, and malignancies. This review provides an overview of basic concept of IL-6 signaling pathway as well as the interplay between IL-6 and renal-resident cells, including podocytes, mesangial cells, endothelial cells, and tubular epithelial cells. Additionally, we summarize the roles of IL-6 in several renal diseases, such as IgA nephropathy, lupus nephritis, diabetic nephropathy, acute kidney injury, and chronic kidney disease.

The role of the hypothalamic-pituitary-adrenal axis in modulating seasonal changes in immunity

Seasonal changes in environmental conditions are accompanied by significant adjustment of multiple biological processes. In temperate regions, the day fraction, or photoperiod, is a robust environmental cue that synchronizes seasonal variations in neuroendocrine and metabolic function. In this work, we propose a semimechanistic mathematical model that considers the influence of seasonal photoperiod changes as well as cellular and molecular adaptations to investigate the seasonality of immune function. Our model predicts that the circadian rhythms of cortisol, our proinflammatory mediator, and its receptor exhibit seasonal differences in amplitude and phase, oscillating at higher amplitudes in the winter season with peak times occurring later in the day. Furthermore, the reduced photoperiod of winter coupled with seasonal alterations in physiological activity induces a more exacerbated immune response to acute stress, simulated in our studies as the administration of an acute dose of endotoxin. Our findings are therefore in accordance with experimental data that reflect the predominance of a proinflammatory state during the winter months. These changes in circadian rhythm dynamics may play a significant role in the seasonality of disease incidence and regulate the diurnal and seasonal variation of disease symptom severity.

Interleukin-6 Receptor Inhibition Prevents Descending Thoracic Aortic Aneurysm Formation

BACKGROUND

Thoracic aortic aneurysms (TAA) and abdominal aortic aneurysms (AAA) represent related but distinct disease processes. Interleukin-6 (IL-6) is known to be significantly upregulated in human TAA and AAA. We hypothesize that loss of IL-6 is protective in experimental TAA and AAA.

METHODS

Murine TAAs or AAAs were created using a novel model in C57/B6 mice by treating the intact aorta with elastase. Cytokine profiles were analyzed with antibody arrays (n = 5 per group). Separately, to determine the role of IL-6, thoracic (n = 7) or abdominal (n = 7) aortas of wild type mice and IL-6 knockout (KO) mice were treated with elastase. Additionally, thoracic animals treated with either the IL-6 receptor antagonist tocilizumab (n = 8) or vehicle (n = 5). Finally, human TAA and AAA were analyzed with human cytokine array.

RESULTS

Elastase treatment of thoracic aortas yielded dilation of 86.8% ± 9.6%, and abdominal aortas produced dilation of 85.6% ± 16.2%. Murine IL-6, CXCL13, and matrix metalloproteinase-9 were significantly elevated in TAA compared with AAA (p = 0.004, 0.028, and 0.001, respectively). The IL-6KO mice demonstrated significantly smaller TAA size relative to wild type mice (wild type 100.1% versus IL-6KO 76.5%, p = 0.04). The IL-6KO mice did not show protection from AAA (p = 0.732). Pharmacologic inhibition of IL-6 resulted in significant reduction in TAA size (tocilizumab 71.5% ± 13.2% versus vehicle 103.6% ± 20.7%, p = 0.005). Human TAA showed significantly greater IL-6 (p < 0.0001) compared with AAA and normal thoracic and abdominal aorta.

CONCLUSIONS

Interleukin-6 is significantly greater in both murine and human TAA compared with AAA, suggesting fundamental differences in these disease processes. Interleukin-6 receptor antagonism attenuates experimental TAA formation, indicating that IL-6 may be a potential target for human thoracic aneurysmal disease.

The soluble interleukin-6 receptor and related proteins

Interleukin-6 is a cytokine involved in the regulation of the immune system and the central nervous system. Interleukin-6 binds to an interleukin-6 receptor, and then associates with a dimer of the ubiquitously expressed gp130 receptor subunit, which initiates intracellular signaling. The interleukin-6 receptor is found in a soluble form, which is generated by proteolytic cleavage and also to a minor extent by translation from an alternatively spliced mRNA. The complex of interleukin-6 bound to the interleukin-6 receptor can stimulate cells, which only express gp130. Such cells are not responsive to interleukin-6 alone. We have for the first time identified the molecular basis of pro-and anti-inflammatory properties of interleukin-6 and we have defined the generation of the soluble IL-6R as a crucial point in the regulation between these two properties. Furthermore, we have deduced a therapeutic principle, which enables us to exclusively block the pro-inflammatory activities of this important cytokine.

IL-6 biology: implications for clinical targeting in rheumatic disease

IL-6 has been linked to numerous diseases associated with inflammation, including rheumatoid arthritis, inflammatory bowel disease, vasculitis and several types of cancer. Moreover, IL-6 is important in the induction of hepatic acute-phase proteins for the trafficking of acute and chronic inflammatory cells, the differentiation of adaptive T-cell responses, and tissue regeneration and homeostatic regulation. Studies have investigated IL-6 biology using cell-bound IL-6 receptors expressed predominantly on hepatocytes and certain haematopoietic cells versus activation mediated by IL-6 and soluble IL-6 receptors via a second protein, gp130, which is expressed throughout the body. Advances in this research elucidating the differential effects of IL-6 activation provide important insights into the role of IL-6 in health and disease, as well as its potential as a therapeutic target. Knowledge of the basic biology of IL-6 and its signalling pathways can better inform both the research agenda for IL-6-based targeted therapies as well as the clinical use of strategies affecting IL-6-mediated inflammation. This Review covers novel, emerging aspects of the biology of IL-6, which might lead to more specific blockade of IL-6 signalling without compromising the protective function of this cytokine in the body's defence against infections.

Role of cytokine signaling during nervous system development

Cytokines are signaling proteins that were first characterized as components of the immune response, but have been found to have pleiotropic effects in diverse aspects of body function in health and disease. They are secreted by numerous cells and are used extensively in intercellular communications to produce different activities, including intricate processes engaged in the ontogenetic development of the brain. This review discusses factors involved in brain growth regulation and recent findings exploring cytokine signaling pathways during development of the central nervous system. In view of existing data suggesting roles for neurotropic cytokines in promoting brain growth and repair, these molecules and their signaling pathways might become targets for therapeutic intervention in neurodegenerative processes due to diseases, toxicity, or trauma.

The IL-6 trans-signaling-STAT3 pathway mediates ECM and cellular proliferation in fibroblasts from hypertrophic scar

The molecular mechanisms behind the pathogenesis of post-burn hypertrophic scar (HS) remain unclear. Here, we investigate the role of interleukin-6 (IL-6) trans-signaling-STAT3 pathway in HS fibroblasts (HSF) derived from burned-induced HS skin. HSF showed increased Tyr 705 STAT3 phosphorylation over normal fibroblast (NF) after IL-6•IL-6Rα stimulation by immunoassays. The endogenous STAT3 target gene, SOCS3, was upregulated in HSF and showed increased STAT3 binding on its promoter relative to NF in Chromatin Immunoprecipitation assay. We observed that the cell surface signaling transducer glycoprotein 130 is upregulated in HSF using Q-RT-PCR and flow cytometry. The production of excessive extracellular matrix (ECM), including the expression of alpha2 (1) procollagen (Col1A2) and fibronectin 1 (FN) were seen in HSFs. A STAT3 peptide inhibitor abrogated FN and Col1A2 gene expression in HSF indicating involvement of STAT3 in ECM production. The cellular proliferation markers Cyclin D1, Bcl-Xl and c-Myc were also upregulated in HSF and knockdown of STAT3 by siRNA attenuated c-Myc expression indicating the essential role of STAT3 in fibroblast proliferation. Taken together, our results suggest that the IL-6-trans-signaling-STAT3 pathway may play an integral role in HS pathogenesis and disruption of this pathway could be a potential therapeutic strategy for the treatment of burn-induced HS.

The regenerative activity of interleukin-6

Interleukin-6 (IL-6) is a cytokine which is involved in many inflammatory processes and in the development of cancer. In addition, IL-6 has been shown to be important for the induction of hepatic acute-phase proteins, for the regeneration of the liver and for the stimulation of B-cells. IL-6 binds to a transmembrane IL-6 receptor (IL-6R), which is present on hepatocytes and some leukocytes. The complex of IL-6 and IL-6R associates with a second protein, gp130, which is expressed on all cells of the body. Since neither IL-6 nor IL-6R has a measurable affinity for gp130, cells, which do not express IL-6R, are not responsive to the cytokine IL-6. It could be shown, however, that a naturally occurring soluble IL-6R (sIL-6R) in complex with IL-6 can bind to gp130 on cells with no IL-6R expression. Therefore, cells shedding the sIL-6R render cells, which only express gp130, responsive to the cytokine. This process has been called trans-signaling. In the present chapter, we summarize the known activities of IL-6 with a special emphasis on regenerative activities, which often depend on the sIL-6R. A designer cytokine called Hyper-IL-6, which is a fusion protein of IL-6 and the sIL-6R, can mimic IL-6 trans-signaling responses in vitro and in vivo with considerably higher efficacy than the combination of the natural proteins IL-6 and sIL-6R. We present recent examples from animal models in which the therapeutic potential of Hyper-IL-6 has been evaluated. We propose that Hyper-IL-6 can be used to induce potent regeneration responses in liver, kidney, and other tissues and therefore will be a novel therapeutic approach in regenerative medicine.

The pro- and anti-inflammatory properties of the cytokine interleukin-6

Interleukin-6 is a cytokine not only involved in inflammation and infection responses but also in the regulation of metabolic, regenerative, and neural processes. In classic signaling, interleukin-6 stimulates target cells via a membrane bound interleukin-6 receptor, which upon ligand binding associates with the signaling receptor protein gp130. Gp130 dimerizes, leading to the activation of Janus kinases and subsequent phosphorylation of tyrosine residues within the cytoplasmic portion of gp130. This leads to the engagement of phosphatase Src homology domains containing tyrosin phosphatase-2 (SHP-2) and activation of the ras/raf/Mitogen-activated protein (MAP) kinase (MAPK) pathway. In addition, signal transducer and activator of transcription factors are recruited, which are phosphorylated, and consequently dimerize whereupon they translocate into the nucleus and activate target genes. Interestingly, only few cells express membrane bound interleukin-6 receptor whereas all cells display gp130 on the cell surface. While cells, which only express gp130, are not responsive to interleukin-6 alone, they can respond to a complex of interleukin-6 bound to a naturally occurring soluble form of the interleukin-6 receptor. Therefore, the generation of soluble form of the interleukin-6 receptor dramatically enlarges the spectrum of interleukin-6 target cells. This process has been named trans-signaling. Here, we review the involvement of both signaling modes in the biology of interleukin-6. It turns out that regenerative or anti-inflammatory activities of interleukin-6 are mediated by classic signaling whereas pro-inflammatory responses of interleukin-6 are rather mediated by trans-signaling. This is important since therapeutic blockade of interleukin-6 by the neutralizing anti-interleukin-6 receptor monoclonal antibody tocilizumab has recently been approved for the treatment of inflammatory diseases. This article is part of a Special Issue entitled: 11th European Symposium on Calcium.

Forced homo- and heterodimerization of all gp130-type receptor complexes leads to constitutive ligand-independent signaling and cytokine-independent growth

We present a novel strategy to enforce cytokine-independent, constitutive signaling of heterodimeric gp130 receptor complexes. Replacing the extracellular domain of gp130-type receptors by IL-15/IL-15R is sufficient to heterodimerize gp130-like receptors and as a consequence leading to sustained cytokine-independent receptor activation.

Naturally ligand independent constitutively active gp130 variants were described to be responsible for inflammatory hepatocellular adenomas. Recently, we genetically engineered a ligand-independent constitutively active gp130 variant based on homodimerization of Jun leucine zippers. Because also heterodimeric complexes within the gp130 family may have tumorigenic potential, we seek to generate ligand-independent constitutively active heterodimers for all known gp130-receptor complexes based on IL-15/IL-15Rα-sushi fusion proteins. Ligand-independent heterodimerization of gp130 with WSX-1, LIFR, and OSMR and of OSMR with GPL led to constitutive, ligand-independent STAT1 and/or STAT3 and ERK1/2 phosphorylation. Moreover, these receptor combinations induced transcription of the STAT3 target genes c-myc and Pim-1 and factor-independent growth of stably transduced Ba/F3-gp130 cells. Here, we establish the IL-15/IL-15Rα-sushi system as a new system to mimic constitutive and ligand-independent activation of homo- and heterodimeric receptor complexes, which might be applicable to other heterodimeric receptor families. A mutated IL-15 protein, which was still able to bind the IL-15Rα-sushi domain, but not to β- and γ-receptor chains, in combination with the 2A peptide technology may be used to translate our in vitro data into the in vivo situation to assess the tumorigenic potential of gp130-heterodimeric receptor complexes.

IL-6-trans-signalling increases rapid-eye-movement sleep in rats

Interleukin 6 (IL-6), a cytokine of the gp130-signalling-family, plays an important role in immediate immunological functions, in metabolism and in the central nervous system. IL-6-signalling is mediated by classic-signalling via the membrane bound IL-6 receptor or by IL-6-trans-signalling via the soluble IL-6 receptor. Whereas the receptor subunit gp130 is ubiquitously expressed within the body, IL-6 receptor expression is restricted to distinct cell populations. Within the brain parenchyma the IL-6 receptor is sparsely expressed, and therefore the brain is mostly dependent on IL-6-trans-signalling in its response to IL-6. Recently we have shown that IL-6-trans-signalling but not classic-signalling plays a pivotal role in the establishment and maintenance of chronic inflammation and cancer, whereas its role in sleep regulation has not been studied so far. We reasoned that the IL-6-trans-signalling mimetic Hyper-IL-6 which in contrast to IL-6 alone can activate almost all cells of the brain might have a profound effect on sleep regulation and performed sleep recordings with rats injected with recombinant Hyper-IL-6. In the present study, the i.c.v. administration of the designer cytokine Hyper-IL-6 into rats at dark onset increased the amount of rapid-eye-movement sleep (REM sleep) but did not affect non-rapid-eye-movement sleep (non-REM sleep). Our data define a new role of IL-6-trans-signalling in sleep regulation.

A new mechanism for the regulation of Gab1 recruitment to the plasma membrane

Adaptor proteins involved in signal transduction fulfil their cellular functions by bringing signalling molecules together and by targeting these signalling components to defined compartments within the cell. Furthermore, adaptor proteins represent a molecular platform from which different signalling pathways are initiated. Gab1 is an adaptor protein that recruits the p85 subunit of the phosphatidylinositol 3-kinase, the adaptor Grb2, the adaptor and phosphatase SHP2 and the GTPase-activating protein Ras-GAP. Gab1 thus contributes to the activation of the PI3K cascade and the MAPK cascade through many growth factors and cytokines. The recruitment of Gab1 to phosphatidylinositol (3,4,5)-trisphosphate within the plasma membrane by its pleckstrin-homology domain is regarded as a major regulatory step for the activation of Gab1. Here, we present a new more complex mechanism for Gab1 translocation that involves and depends on the activation of ERK. We demonstrate that the presence of PI3K activity in the cell is not sufficient for binding Gab1 to the plasma membrane. Instead, additional MAPK-dependent phosphorylation of Ser551 in Gab1 is crucial for the recruitment of Gab1 to the plasma membrane. This mechanism represents a new mode of regulation for the function of PH domains.

Prostaglandin E1 inhibits IL-6-induced MCP-1 expression by interfering specifically in IL-6-dependent ERK1/2, but not STAT3, activation

IL (interleukin)-6 exerts pro- as well as anti-inflammatory activities. Beside many other activities, IL-6 is the major inducer of acute phase proteins in the liver, acts as a differentiation factor for blood cells, as migration factor for T-cells and is a potent inducer of the chemokine MCP-1 (monocyte chemoattractant protein-1). Recent studies have focused on the negative regulation of IL-6 signal transduction through the IL-6-induced feedback inhibitors SOCS (suppressor of cytokine signalling) 1 and SOCS3 or the protein tyrosine phosphatases SHP-2 (Src homology 2 domain-containing protein tyrosine phosphatase 2) and TcPTP (T-cell protein tyrosine phosphatase). Studies on the cross-talk between pro-inflammatory mediators (IL-1, tumour necrosis factor, lipopolysaccharide) and IL-6 elucidated further regulatory mechanisms. Less is known about the regulation of IL-6 signal transduction by hormone/cytokine signalling through G-protein-coupled receptors. This is particularly surprising since many of these hormones (such as prostaglandins and chemokines) play an important role in inflammatory processes. In the present study, we have investigated the inhibitory activity of PGE(1) (prostaglandin E(1)) on IL-6-induced MCP-1 expression and have elucidated the underlying molecular mechanism. Surprisingly, PGE(1) does not affect IL-6-induced STAT (signal transducer and activator of transcription) 3 activation, but does affect ERK (extracellular-signal-regulated kinase) 1/2 activation which is crucial for IL-6-dependent expression of MCP-1. In summary, we have discovered a specific cross-talk between the adenylate cyclase cascade and the IL-6-induced MAPK (mitogen-activated protein kinase) cascade and have investigated its impact on IL-6-dependent gene expression.

SHPS-1/SIRP1alpha contributes to interleukin-6 signalling

The transmembrane glycoprotein signal regulatory protein/SHP2-substrate (SIRP1alpha/SHPS-1) has been implicated in growth factor- and cell adhesion-induced signalling. Here we report on the contribution of SIRP1alpha to IL-6 type cytokine signalling. SIRP1alpha binds the protein tyrosine phosphatase SHP2 upon treatment with interleukin-6 in a stimulation-dependent manner. Mouse embryonic fibroblasts expressing a SIRP1alpha protein which lacks the intracellular part show enhanced SHP2 phosphorylation and ERK1/2 activation in response to IL-6, suggesting that SIRP1alpha affects IL-6-signalling through SHP2. Whereas SHP2 phosphorylation is enhanced in SIRP1alpha-deficient cells STAT3 activation is delayed and STAT3-dependent gene induction is reduced which correlates with reduced STAT3 serine phosphorylation. Our results indicate that SIRP1alpha contributes to IL-6 signalling by counteracting SHP2 phosphorylation which consequently affects ERK-activation and STAT3-dependent transactivation as well as target gene expression. Our observations will help to understand the tight balance of MAPK- and STAT3-activation in response to IL-6 which was found to be misbalanced in many autoimmune diseases, inflammatory proliferative diseases and cancer.

Conservation of IL-6 trans-signaling mechanisms controlling L-selectin adhesion by fever-range thermal stress

Fever is associated with improved survival during infection in endothermic and ectothermic species although the protective mechanisms are largely undefined. Previous studies indicate that fever-range thermal stress increases the binding activity of the L-selectin homing receptor in human or mouse leukocytes, thereby promoting trafficking to lymphoid tissues across high endothelial venules (HEV). Here, we examined the evolutionary conservation of thermal regulation of L-selectin-like adhesion. Leukocytes from animals representing four taxa of vertebrates (mammals, avians, amphibians, teleosts) were shown to mediate L-selectin-like adhesion under shear to MECA-79-reactive ligands on mouse HEV in cross-species in vitro adherence assays. L-selectin-like binding activity was markedly increased by fever-range thermal stress in leukocytes of all species examined. Comparable increases in L-selectin-like adhesion were induced by thermal stress, IL-6, or the IL-6/soluble IL-6 receptor fusion protein, hyper-IL-6. Analysis of the molecular basis of thermal regulation of L-selectin-like adhesion identified a common IL-6 trans-signaling mechanism in endotherms and ectotherms that resulted in activation of JAK/STAT signaling and was inhibited by IL-6 neutralizing antibodies or recombinant soluble gp130. Conservation of IL-6-dependent mechanisms controlling L-selectin adhesion over hundreds of millions of years of vertebrate evolution strongly suggests that this is a beneficial focal point regulating immune surveillance during febrile inflammatory responses.

Regulation of a lymphocyte-endothelial-IL-6 trans-signaling axis by fever-range thermal stress: hot spot of immune surveillance

The pleiotropic cytokine, interleukin-6 (IL-6), has emerged in recent years as a key regulator of the transition from innate to adaptive immunity through its ability to modulate leukocyte recruitment at inflammatory sites. This review highlights a newly identified role for IL-6 trans-signaling, initiated by an agonistic complex of IL-6 and a soluble form of IL-6 receptor alpha, in heightening immune surveillance of peripheral lymphoid organs during febrile inflammatory responses. Inflammatory cues provided by the thermal component of fever trigger IL-6 trans-signaling to act at discrete levels in the multistep adhesion cascade that governs the entry of blood-borne lymphocytes across 'gatekeeper' high endothelial venules (HEVs) in lymph nodes and Peyer patches. IL-6 trans-signaling-dependent mechanisms have been elucidated during thermal stimulation of primary tethering and rolling of lymphocytes along the lumenal surface of HEVs as well as during secondary firm arrest of lymphocytes in HEVs prior to their migration into the underlying parenchyma. These mechanisms profoundly increase the probability that lymphocytes that continuously patrol the body will engage in productive encounters with target antigens sequestered within lymphoid organs. Findings that the lymphocyte-HEV-IL-6 trans-signaling biological axis functions as a thermally-sensitive alert system that promotes immune surveillance provide insight into one of the unresolved mysteries in immunology regarding the benefits of mounting a febrile reaction during inflammation.

Interleukin 27 induces differentiation of neural C6-precursor cells into astrocytes

Interleukin 6 (IL6)-type cytokines are major regulators of inflammation and thereby contribute to the neuropathology and pathophysiology associated with inflammation of the central nervous system (CNS). Furthermore, astrocyte development which is a key process in the development of the CNS is also controlled by cytokines of the IL6-family. Interleukin 27 (IL27) is a recently identified member of this family and has been implicated in the inhibition of TH17 T-cell-responses. Here we show that IL27 and the HHV8 encoded viral IL6 (vIL6) induce C6 glioma cells to differentiate into an astrocyte-like state. Cytokine stimulation led to STAT-factor phosphorylation and consequently to protein expression of the astrocyte marker glial fibrillary acidic protein (GFAP). These data could be confirmed by GFAP-immunostaining of stimulated cells. Taken together, IL27 and vIL6 can be considered as new astrocyte-inducing cytokines of the brain.

Interleukin-6 and its receptor: from bench to bedside

Interleukin-6 (IL-6) is an inflammatory cytokine with a well-documented role in inflammation and cancer. The cytokine binds to a membrane bound IL-6 receptor (IL-6R) and this complex associates with two molecules of the signal transducing protein gp130 thereby initiating intracellular signaling. While gp130 is present on most if not all cells of the body, the IL-6R is only present on some cells, mainly hepatocytes and several leukocytes. Cells, which only express gp130 and no IL-6R are refractory to IL-6 signals. We have shown earlier that the IL-6R can exist as a soluble protein generated by limited proteolysis of the membrane bound receptor or by translation from an alternatively spliced mRNA. This soluble IL-6R (sIL-6R) can bind the ligand IL-6 and the soluble complex of sIL-6R and IL-6 can bind to gp130 on cells which lack the membrane bound IL-6R and trigger gp130 signaling. We have named this process 'trans-signaling'. We will review data, which clearly show that IL-6 uses classical signaling via the membrane bound receptor and trans-signaling via the soluble receptor in various physiological and pathophysiological situations. Furthermore, we have developed designer cytokines, which can specifically enhance or inhibit IL-6 trans-signaling. These designer cytokines have been shown to be extremely useful to in therapeutic applications ranging from the long-term culture of stem cells and enhancing liver regeneration up to the blockade of chronic inflammation and cancer.