Purification and characterization of the soluble interleukin-6 receptor from human plasma and identification of an isoform generated through alternative splicing

EGFR stimulation enables IL-6 trans-signalling via iRhom2-dependent ADAM17 activation in mammary epithelial cells

The cytokine interleukin-6 (IL-6) has considerable pro-inflammatory properties and is a driver of many physiological and pathophysiological processes. Cellular responses to IL-6 are mediated by membrane-bound or soluble forms of the IL-6 receptor (IL-6R) complexed with the signal-transducing subunit gp130. While expression of the membrane-bound IL-6R is restricted to selected cell types, soluble IL-6R (sIL-6R) enables gp130 engagement on all cells, a process termed IL-6 trans-signalling and considered to be pro-inflammatory. sIL-6R is predominantly generated through proteolytic processing by the metalloproteinase ADAM17. ADAM17 also liberates ligands of the epidermal growth factor receptor (EGFR), which is a prerequisite for EGFR activation and results in stimulation of proliferative signals. Hyperactivation of EGFR mostly due to activating mutations drives cancer development. Here, we reveal an important link between overshooting EGFR signalling and the IL-6 trans-signalling pathway. In epithelial cells, EGFR activity induces not only IL-6 expression but also the proteolytic release of sIL-6R from the cell membrane by increasing ADAM17 surface activity. We find that this derives from the transcriptional upregulation of iRhom2, a crucial regulator of ADAM17 trafficking and activation, upon EGFR engagement, which results in increased surface localization of ADAM17. Also, phosphorylation of the EGFR-downstream mediator ERK mediates ADAM17 activity via interaction with iRhom2. In sum, our study reveals an unforeseen interplay between EGFR activation and IL-6 trans-signalling, which has been shown to be fundamental in inflammation and cancer.

Functional role of skeletal muscle-derived interleukin-6 and its effects on lipid metabolism

The detrimental impact of obesity on human health is increasingly evident with the rise in obesity-related diseases. Skeletal muscle, the crucial organ responsible for energy balance metabolism, plays a significant role as a secretory organ by releasing various myokines. Among these myokines, interleukin 6 (IL-6) is closely associated with skeletal muscle contraction. IL-6 triggers the process of lipolysis by mobilizing energy-storing adipose tissue, thereby providing energy for physical exercise. This phenomenon also elucidates the health benefits of regular exercise. However, skeletal muscle and adipose tissue maintain a constant interaction, both directly and indirectly. Direct interaction occurs through the accumulation of excess fat within skeletal muscle, known as ectopic fat deposition. Indirect interaction takes place when adipose tissue is mobilized to supply the energy for skeletal muscle during exercise. Consequently, maintaining a functional balance between skeletal muscle and adipose tissue becomes paramount in regulating energy metabolism and promoting overall health. IL-6, as a representative cytokine, participates in various inflammatory responses, including non-classical inflammatory responses such as adipogenesis. Skeletal muscle influences adipogenesis through paracrine mechanisms, primarily by secreting IL-6. In this research paper, we aim to review the role of skeletal muscle-derived IL-6 in lipid metabolism and other physiological activities, such as insulin resistance and glucose tolerance. By doing so, we provide valuable insights into the regulatory function of skeletal muscle-derived myokines in lipid metabolism.

IL-6/JAK/STAT3 Signaling in Breast Cancer Metastasis: Biology and Treatment

Breast cancer is the most commonly diagnosed cancer in women. Metastasis is the primary cause of mortality for breast cancer patients. Multiple mechanisms underlie breast cancer metastatic dissemination, including the interleukin-6 (IL-6)-mediated signaling pathway. IL-6 is a pleiotropic cytokine that plays an important role in multiple physiological processes including cell proliferation, immune surveillance, acute inflammation, metabolism, and bone remodeling. IL-6 binds to the IL-6 receptor (IL-6Rα) which subsequently binds to the glycoprotein 130 (gp130) receptor creating a signal transducing hexameric receptor complex. Janus kinases (JAKs) are recruited and activated; activated JAKs, in turn, phosphorylate signal transducer and activator of transcription 3 (STAT3) for activation, leading to gene regulation. Constitutively active IL-6/JAK/STAT3 signaling drives cancer cell proliferation and invasiveness while suppressing apoptosis, and STAT3 enhances IL-6 signaling to promote a vicious inflammatory loop. Aberrant expression of IL-6 occurs in multiple cancer types and is associated with poor clinical prognosis and metastasis. In breast cancer, the IL-6 pathway is frequently activated, which can promote breast cancer metastasis while simultaneously suppressing the anti-tumor immune response. Given these important roles in human cancers, multiple components of the IL-6 pathway are promising targets for cancer therapeutics and are currently being evaluated preclinically and clinically for breast cancer. This review covers the current biological understanding of the IL-6 signaling pathway and its impact on breast cancer metastasis, as well as, therapeutic interventions that target components of the IL-6 pathway including: IL-6, IL-6Rα, gp130 receptor, JAKs, and STAT3.

Function and proteolytic generation of the soluble interleukin-6 receptor in health and disease

The pleiotropic cytokine interleukin-6 (IL-6) is involved in numerous physiological and pathophysiological functions that include development, immune cell differentiation, inflammation and cancer. IL-6 can signal via the membrane-bound IL-6 receptor (IL-6R, classic signaling) or via soluble forms of the IL-6R (sIL-6R, trans-signaling). Both modes of signaling induce the formation of a homodimer of the signal transducing β-receptor glycoprotein 130 (gp130) and the activation of several intracellular signaling cascades, e.g. the Jak/STAT pathway. Intriguingly, only IL-6 trans-signaling is required for the pro-inflammatory properties of IL-6, while regenerative and anti-inflammatory functions are mediated via classic signaling. The sIL-6R is generated by different molecular mechanisms, including alternative mRNA splicing, proteolysis of the membrane-bound IL-6R and the release of extracellular vesicles. In this review, we give an in-depth overview on these molecular mechanisms with a special emphasize on IL-6R cleavage by the metalloprotease ADAM17 and other proteases. We discuss the biological functions of the sIL-6R and highlight attempts to selectively block IL-6 trans-signaling in pre-clinical animal models as well as in clinical studies in patients with inflammatory bowel disease.

The role of proteolysis in interleukin-11 signaling

Although interleukin-11 (IL-11) was discovered more than 30 years ago, it remains an understudied member of the IL-6 family of cytokines. While it was originally discovered as a secreted factor that could foster megakaryocyte maturation and was therefore used as a recombinant protein to increase platelet production in patients with thrombocytopenia, recent research has established important roles for IL-11 in inflammation, fibrosis and cancer. In order to initiate signal transduction, IL-11 binds first to a non-signaling membrane-bound IL-11 receptor (IL-11R, classic signaling), which subsequently induces the formation of a heterodimer of the signal-transducing receptor gp130 that is shared with the other family members. Complex formation initiates several intracellular signaling cascades, most notably the Janus kinase/Signal Transducer and Activator of Transcription (Jak/STAT) pathway. We have recently identified a trans-signaling mechanism, in which IL-11 binds to soluble forms of the IL-11R (sIL-11R) and the agonistic IL-11/sIL-11R complex can activate cells that do not express the IL-11R and would usually not respond to IL-11. The generation of sIL-11R and thus the initiation of IL-11 trans-signaling is mediated by proteolytic cleavage. In this review, we summarize the current state of knowledge regarding IL-11R cleavage, highlight recent developments in IL-11 biology and discuss therapeutic opportunities and challenges in the light of IL-11 classic and trans-signaling.

Role of IL-6 signalling in Polycystic Ovarian Syndrome associated inflammation

Polycystic ovarian syndrome is a major factor contributing to the increasing incidence of infertility around the world. The metabolic effects of prolonged exposure to PCOS have been well determined from previous studies and seem to be detrimental in the long run. Studies have shown a multitude of conditions like insulin resistance, obesity and cardiovascular disorders to be associated with PCOS. This makes PCOS a syndrome requiring utmost attention in terms of women's health and reproduction. Interleukin-6 is one of the many cytokines released by adipocytes from fat deposits in the body. This review focuses on the Interleukin 6 signaling pathway and the data available on inflammatory modulators due to its significance PCOS mediated inflammation. There is noteworthy evidence of elevated IL-6 concentration in PCOS subjects that has been discussed in detail. The regulation of IL-6 levels in the body is in turn, maintained by a close relation with other cytokines, especially by a key regulator, NF-κB. Being involved in a multitude of other pathological conditions like rheumatoid arthritis, cardiovascular disorders, asthma, colon cancer and many more, the role of IL-6 is also investigated in PCOS in search of a probable reason for underlying inflammatory condition. On summarizing the IL-6 signaling pathway and therapeutic exploitation of the same, we see that IL-6 targeted drugs may be an efficient way of treating PCOS associated inflammation.

Modelling Macular Edema: The Effect of IL-6 and IL-6R Blockade on Human Blood-Retinal Barrier Integrity In Vitro

Purpose

Macular edema (ME) is a leading cause of visual loss in a range of retinal diseases and despite the use of antivascular endothelial growth factor (anti-VEGF) agents, its successful treatment remains a major clinical challenge. Based on the indirect clinical evidence that interleukin-6 (IL-6) is a key additional candidate mediator of ME, we interrogated the effect of IL-6 on blood–retinal barrier (BRB) integrity in vitro.

Methods

Human retinal pigment epithelial cell (ARPE-19) and human retinal microvascular endothelial cell (HRMEC) monolayers were used to mimic the outer and inner BRB, respectively. Their paracellular permeability was assessed by measuring the passive permeation of 40 kDa fluorescein isothiocyanate (FITC)-dextran across confluent cells in the presence of IL-6. Transendothelial/epithelial electrical resistance (TEER) then was measured and the distribution of the tight junction protein ZO-1 was assessed by immunofluorescence using confocal microscopy.

Results

Treatment with IL-6 for 48 hours significantly increased the diffusion rate of FITC-dextran, decreased TEER, and disrupted the distribution of ZO-1 in ARPE-19 cells, which constitutively express the IL-6 transmembrane receptor, and this was reversed with IL-6R blockade. In contrast, IL-6 did not affect the paracellular permeability, TEER, or ZO-1 distribution in HRMECs.

Conclusions

These in vitro data support the hypothesis that IL-6 reversibly disrupts the integrity of ARPE-19 cells, but it does not affect HRMECs.

Translational Relevance

IL-6 is a candidate therapeutic target in the treatment of outer BRB driven ME.

Response to IL-6 trans- and IL-6 classic signalling is determined by the ratio of the IL-6 receptor α to gp130 expression: fusing experimental insights and dynamic modelling

BACKGROUND

Interleukin-6 is a pleiotropic cytokine with high clinical relevance and an important mediator of cellular communication, orchestrating both pro- and anti-inflammatory processes. Interleukin-6-induced signalling is initiated by binding of IL-6 to the IL-6 receptor α and subsequent binding to the signal transducing receptor subunit gp130. This active receptor complex initiates signalling through the Janus kinase/signal transducer and activator of transcription pathway. Of note, IL-6 receptor α exists in a soluble and a transmembrane form. Binding of IL-6 to membrane-bound IL-6 receptor α induces anti-inflammatory classic signalling, whereas binding of IL-6 to soluble IL-6 receptor α induces pro-inflammatory trans-signalling. Trans-signalling has been described to be markedly stronger than classic signalling. Understanding the molecular mechanisms that drive differences between trans- and classic signalling is important for the design of trans-signalling-specific therapies. These differences will be addressed here using a combination of dynamic mathematical modelling and molecular biology.

METHODS

We apply an iterative systems biology approach using set-based modelling and validation approaches combined with quantitative biochemical and cell biological analyses.

RESULTS

The combination of experimental analyses and dynamic modelling allows to relate the observed differences between IL-6-induced trans- and classic signalling to cell-type specific differences in the expression and ratios of the individual subunits of the IL-6 receptor complex. Canonical intracellular Jak/STAT signalling is indifferent in IL-6-induced trans- and classic signalling.

CONCLUSION

This study contributes to the understanding of molecular mechanisms of IL-6 signal transduction and underlines the power of combined dynamical modelling, model-based validation and biological experiments. The opposing pro- and anti-inflammatory responses initiated by IL-6 trans- and classic signalling depend solely on the expression ratios of the subunits of the entire receptor complex. By pointing out the importance of the receptor expression ratio for the strength of IL-6 signalling this study lays a foundation for future precision medicine approaches that aim to selectively block pro-inflammatory trans-signalling. Furthermore, the derived models can be used for future therapy design.

The metalloprotease ADAM17 in inflammation and cancer

Proteolytic cleavage of transmembrane proteins is an important post-translational modification that regulates the biological function of numerous transmembrane proteins. Among the 560 proteases encoded in the human genome, the metalloprotease A Disintegrin and Metalloprotease 17 (ADAM17) has gained much attention in recent years and has emerged as a central regulatory hub in inflammation, immunity and cancer development. In order to do so, ADAM17 cleaves a variety of substrates, among them the interleukin-6 receptor (IL-6R), the pro-inflammatory cytokine tumor necrosis factor α (TNFα) and most ligands of the epidermal growth factor receptor (EGFR). This review article provides an overview of the functions of ADAM17 with a special focus on its cellular regulation. It highlights the importance of ADAM17 to understand the biology of IL-6 and TNFα and their role in inflammatory diseases. Finally, the role of ADAM17 in the formation and progression of different tumor entities is discussed.

Linking energy sensing to suppression of JAK-STAT signalling: A potential route for repurposing AMPK activators?

Exaggerated Janus kinase-signal transducer and activator of transcription (JAK-STAT) signalling is key to the pathogenesis of pro-inflammatory disorders, such as rheumatoid arthritis and cardiovascular diseases. Mutational activation of JAKs is also responsible for several haematological malignancies, including myeloproliferative neoplasms and acute lymphoblastic leukaemia. Accumulating evidence links adenosine 5'-monophosphate (AMP)-activated protein kinase (AMPK), an energy sensor and regulator of organismal and cellular metabolism, with the suppression of immune and inflammatory processes. Recent studies have shown that activation of AMPK can limit JAK-STAT-dependent signalling pathways via several mechanisms. These novel findings support AMPK activation as a strategy for management of an array of disorders characterised by hyper-activation of the JAK-STAT pathway. This review discusses the pivotal role of JAK-STAT signalling in a range of disorders and how both established clinically used and novel AMPK activators might be used to treat these conditions.

Proteolytic Origin of the Soluble Human IL-6R In Vivo and a Decisive Role of N-Glycosylation

Signaling of the cytokine interleukin-6 (IL-6) via its soluble IL-6 receptor (sIL-6R) is responsible for the proinflammatory properties of IL-6 and constitutes an attractive therapeutic target, but how the sIL-6R is generated in vivo remains largely unclear. Here, we use liquid chromatography–mass spectrometry to identify an sIL-6R form in human serum that originates from proteolytic cleavage, map its cleavage site between Pro-355 and Val-356, and determine the occupancy of all O- and N-glycosylation sites of the human sIL-6R. The metalloprotease a disintegrin and metalloproteinase 17 (ADAM17) uses this cleavage site in vitro, and mutation of Val-356 is sufficient to completely abrogate IL-6R proteolysis. N- and O-glycosylation were dispensable for signaling of the IL-6R, but proteolysis was orchestrated by an N- and O-glycosylated sequon near the cleavage site and an N-glycan exosite in domain D1. Proteolysis of an IL-6R completely devoid of glycans is significantly impaired. Thus, glycosylation is an important regulator for sIL-6R generation.

Interleukin-6 (IL-6) is a cytokine secreted by our body upon infection or trauma to stimulate the immune system response. IL-6 is partially responsible for fever and triggers inflammation in many diseases. It activates its target cells via the membrane-bound IL-6 receptor (IL-6R), and soluble forms of this receptor (sIL-6R) are present in high amounts in the serum of healthy individuals and mediate the inflammatory response in all cells of the human body. However, it remains unclear how the soluble form of this cytokine is generated in humans. In this study, we isolate sIL-6R from human serum and show that the majority is produced via cleavage of the membrane-bound IL-6R by a protease. We identify the exact cleavage site and find that it is identical to a cleavage site used by the metalloprotease ADAM17. We further show that glycosylation, a post-transcriptional modification, is dispensable for the transport and biological function of IL-6R and map the occupancy of all O- and N-glycosylation sites. However, we find that only a single N-glycan is critically involved in the regulation of proteolysis by ADAM17 and conclude that glycosylation is an important regulator for sIL-6R generation.

Generation of Soluble Interleukin-11 and Interleukin-6 Receptors: A Crucial Function for Proteases during Inflammation

The cytokines interleukin-11 (IL-11) and IL-6 are important proteins with well-defined pro- and anti-inflammatory functions. They activate intracellular signaling cascades through a homodimer of the ubiquitously expressed signal-transducing β-receptor glycoprotein 130 (gp130). Specificity is gained through the cell- and tissue-specific expression of the nonsignaling IL-11 and IL-6 α-receptors (IL-11R and IL-6R), which determine the responsiveness of the cell to these two cytokines. IL-6 is a rare example, where its soluble receptor (sIL-6R) has agonistic properties, so that the IL-6/sIL-6R complex is able to activate cells that are usually not responsive to IL-6 alone (trans-signaling). Recent evidence suggests that IL-11 can signal via a similar trans-signaling mechanism. In this review, we highlight similarities and differences in the functions of IL-11 and IL-6. We summarize current knowledge about the generation of the sIL-6R and sIL-11R by different proteases and discuss possible roles during inflammatory processes. Finally, we focus on the selective and/or combined inhibition of IL-6 and IL-11 signaling and how this might translate into the clinics.

Shedding of Endogenous Interleukin-6 Receptor (IL-6R) Is Governed by A Disintegrin and Metalloproteinase (ADAM) Proteases while a Full-length IL-6R Isoform Localizes to Circulating Microvesicles

Generation of the soluble interleukin-6 receptor (sIL-6R) is a prerequisite for pathogenic IL-6 trans-signaling, which constitutes a distinct signaling pathway of the pleiotropic cytokine interleukin-6 (IL-6). Although in vitro experiments using ectopically overexpressed IL-6R and candidate proteases revealed major roles for the metalloproteinases ADAM10 and ADAM17 in IL-6R shedding, the identity of the protease(s) cleaving IL-6R in more physiological settings, or even in vivo, remains unknown. By taking advantage of specific pharmacological inhibitors and primary cells from ADAM-deficient mice we established that endogenous IL-6R of both human and murine origin is shed by ADAM17 in an induced manner, whereas constitutive release of endogenous IL-6R is largely mediated by ADAM10. Although circulating IL-6R levels are altered in various diseases, the origin of blood-borne IL-6R is still poorly understood. It has been shown previously that ADAM17 hypomorphic mice exhibit unaltered levels of serum sIL-6R. Here, by quantification of serum sIL-6R in protease-deficient mice as well as human patients we also excluded ADAM10, ADAM8, neutrophil elastase, cathepsin G, and proteinase 3 from contributing to circulating sIL-6R. Furthermore, we ruled out alternative splicing of the IL-6R mRNA as a potential source of circulating sIL-6R in the mouse. Instead, we found full-length IL-6R on circulating microvesicles, establishing microvesicle release as a novel mechanism for sIL-6R generation.

Interleukin-6 and its receptors: a highly regulated and dynamic system

Interleukin-6 (IL-6) is a multifunctional cytokine with well-defined pro- and anti-inflammatory properties. Although only small amounts in the picogram range can be detected in healthy humans, IL-6 expression is highly and transiently up-regulated in nearly all pathophysiological states. IL-6 induces intracellular signaling pathways after binding to its membrane-bound receptor (IL-6R), which is only expressed on hepatocytes and certain subpopulations of leukocytes (classic signaling). Transduction of the signal is mediated by the membrane-bound β-receptor glycoprotein 130 (gp130). In a second pathway, named trans-signaling, IL-6 binds to soluble forms of the IL-6R (sIL-6R), and this agonistic IL-6/sIL-6R complexes can in principle activate all cells due to the uniform expression of gp130. Importantly, several soluble forms of gp130 (sgp130) are found in the human blood, which are considered to be the natural inhibitors of IL-6 trans-signaling. Most pro-inflammatory roles of IL-6 have been attributed to the trans-signaling pathway, whereas anti-inflammatory and regenerative signaling, including the anti-bacterial acute phase response of the liver, is mediated by IL-6 classic signaling. In this simplistic view, only a minority of cell types expresses the IL-6R and is therefore responsive for IL-6 classic signaling, whereas gp130 is ubiquitously expressed throughout the human body. However, several reports point towards a much more complex situation. A plethora of factors, including proteases, cytokines, chemical drugs, and intracellular signaling pathways, are able to modulate the cellular expression of the membrane-bound and soluble forms of IL-6R and gp130. In this review, we summarize current knowledge of regulatory mechanisms that control and regulate the dynamic expression of IL-6 and its two receptors.

The contribution of the functional IL6R polymorphism rs2228145, eQTLs and other genome-wide SNPs to the heritability of plasma sIL-6R levels

The non-synonymous SNP rs2228145 in the IL6R gene on chromosome 1q21.3 is associated with a wide range of common diseases, including asthma, rheumatoid arthritis, type 1 diabetes and coronary heart disease. We examined the contribution of this functional IL6R gene polymorphism rs2228145 versus other genome-wide SNPs to the variance of sIL-6R levels in blood plasma in a large population-based sample (N ~5,000), and conducted an expression QTL analysis to identify SNPs associated with IL6R gene expression. Based on data from 2,360 twin families, the broad heritability of sIL-6R was estimated at 72 and 51% of the total variance was explained by the functional SNP rs2228145. Converging findings from GWAS, linkage, and GCTA analyses indicate that additional variance of sIL-6R levels can be explained by other variants in the IL6R region, including variants at the 3'-end of IL6R tagged by rs60760897 that are associated with IL6R RNA expression.

IL6 trans-signaling promotes functional recovery of hypofunctional phagocytes through STAT3 activation during peritonitis

OBJECTIVE

The role of high interleukin 6 (IL6) levels has not been clearly explained in severe sepsis. We show that the augmentation of the IL6 signal by recombinant IL6 receptors (rIL6R) delivery allows the functional recovery of phagocytes in a peritonitis mouse model.

MATERIALS AND METHODS

Mice were challenged intraperitoneally (i.p.) with live Staphylococcus aureus for effect of IL6R delivery on the 24 h-survival, bacterial clearance and cellular responses. In additional experiments to assess the effect of IL6R delivery on phagocytosis, the model was i.p. inoculated with heat-killed S. aureus with or without rIL6R and the peritoneal lavage fluid and cells were collected at 1 h after the i.p. inoculation of S. aureus.

RESULTS

The IL6R delivery tended to improve 24 h survival and increase bacteria clearance from the septic mice. The rIL6R treatment to heat-killed bacteria challenged mice augmented the uptake of bacteria and phagosome acidification, inducing the phosphorylation of STAT3 in peritoneal cells within 1 h after the IL6R delivery. Furthermore, the rIL6R delivery prevented the extracellular release of neutrophil elastase activity and myeloperoxidase (harmful factors).

CONCLUSIONS

These results indicate that augmentation of IL6 signaling appears to be critical for the effective management of hypofunctional neutrophils during severe inflammation, such as sepsis.

Plasticity and cross-talk of interleukin 6-type cytokines

Interleukin (IL)-6-type cytokines are critically involved in health and disease. The duration and strength of IL-6-type cytokine-mediated signaling is tightly regulated to avoid overshooting activities. Here, molecular mechanisms of inter-familiar cytokine cross-talk are reviewed which regulate dynamics and strength of IL-6 signal transduction. Both plasticity and cytokine cross-talk are significantly involved in pro- and anti-inflammatory/regenerative properties of IL-6-type cytokines. Furthermore, we focus on IL-6-type cytokine/cytokine receptor plasticity and cross-talk exemplified by the recently identified composite cytokines IL-30/IL-6R and IL-35, the first inter-familiar IL-6/IL-12 family member. The complete understanding of the intra- and extracellular cytokine networks will aid to develop novel tailor-made therapeutic strategies with reduced side effects.

Extending the dynamic range of label-free mass spectrometric quantification of affinity purifications

Affinity purification (AP) of protein complexes combined with LC-MS/MS analysis is the current method of choice for identification of protein-protein interactions. Their interpretation with respect to significance, specificity, and selectivity requires quantification methods coping with enrichment factors of more than 1000-fold, variable amounts of total protein, and low abundant, unlabeled samples. We used standardized samples (0.1-1000 fmol) measured on high resolution hybrid linear ion trap instruments (LTQ-FT/Orbitrap) to characterize and improve linearity and dynamic range of label-free approaches. Quantification based on spectral counts was limited by saturation and ion suppression effects with samples exceeding 100 ng of protein, depending on the instrument setup. In contrast, signal intensities of peptides (peak volumes) selected by a novel correlation-based method (TopCorr-PV) were linear over at least 4 orders of magnitude and allowed for accurate relative quantification of standard proteins spiked into a complex protein background. Application of this procedure to APs of the voltage-gated potassium channel Kv1.1 as a model membrane protein complex unambiguously identified the whole set of known interaction partners together with novel candidates. In addition to discriminating these proteins from background, we could determine efficiency, cross-reactivities, and selection biases of the used purification antibodies. The enhanced dynamic range of the developed quantification procedure appears well suited for sensitive identification of specific protein-protein interactions, detection of antibody-related artifacts, and optimization of AP conditions.

The soluble Interleukin 6 receptor: generation and role in inflammation and cancer

Soluble cytokine receptors are frequently found in human serum, most of them possessing antagonistic properties. The Interleukin 6 receptor (IL-6R) is found as a transmembrane protein on hepatocytes and subsets of leukocytes, but soluble isoforms of the IL-6R (sIL-6R) are generated by alternative splicing or by limited proteolysis of the ADisintegrin And Metalloproteinases (ADAM) gene family members ADAM10 and ADAM17. Importantly, the sIL-6R in complex with its ligand Interleukin 6 (IL-6) has agonistic functions and requires cells expressing the signal transducing ß-receptor gp130 but not the membrane-bound IL-6R. We have called this process IL-6 trans-signaling. Naturally occurring isoforms of soluble gp130 (sgp130), which are generated by alternative splicing, are natural inhibitors of IL-6 trans-signaling, leaving IL-6 classic signaling via the membrane-bound IL-6R unaffected. We used recombinant sgp130Fc protein and recently generated transgenic mice expressing high levels of sgp130Fc to discriminate between classic and trans-signaling in vivo, and demonstrated that IL-6 trans-signaling is critically involved in generation and maintenance of several inflammatory and autoimmune diseases including chronic inflammatory bowel disease, rheumatoid arthritis, peritonitis and asthma, as well as inflammation-induced colon cancer.

Cytokines in idiopathic inflammatory myopathies

Recent findings suggest cytokines as important key molecules in the pathogenic mechanisms of idiopathic inflammatory myopathies, myositis. In this review, we focus on cytokines with a potential role in disease mechanisms in myositis and present some general information on individual cytokines and an updated summary from the literature concerning cytokines in these disorders. The idiopathic inflammatory myopathies is a heterogeneous group of disorders clinically characterized by symmetric proximal muscle weakness and by certain defined histolopathological findings, including inflammatory infiltrates in muscle tissue. Other prominent findings in the target tissue of these patients are defined molecular changes of blood vessels and muscle fibers, including reformation to high endothelial venule (HEV)-like blood vessels and intensive MHC class I expression in muscle fibers. The predominant clinical symptoms of muscle weakness and decreased muscle endurance are shared by all subsets of inflammatory myopathies and indicate that some pathogenic mechanisms related to muscle function may be shared by the different disease groups. Studies on cytokine gene, RNA and protein expression in muscle tissue from patients with various forms of the disease also indicate similar profiles, despite different phenotypes of the inflammatory cells present in muscle tissue from the different subsets of myositis. There is a pronounced expression of various cytokines in muscle tissue, among which the proinflammatory cytokines TNF-alpha and IL-1 are most widely explored in the inflammatory myopathies, which has made them into potential therapeutic targets. The use of targeted cytokine therapy has been successful in several other chronic inflammatory diseases and although the exact role of cytokines in chronic idiopathic inflammatory myopathies remains to be delineated their potential role as targets for new therapies in this disorder will be discussed in this review.

IL6/sIL6R complex contributes to emergency granulopoietic responses in G-CSF- and GM-CSF-deficient mice

Mice defective in both granulocyte colony-stimulating factor (G-CSF) and granulocyte-macrophage colony-stimulating factor (GM-CSF) have severely impaired neutrophil production and function, yet these mice respond to acute pathogen challenge with a significant neutrophil response. We have recently reported the development of an in vitro system to detect granulopoietic cytokines secreted from cells isolated from G-CSF, GM-CSF double knockout mice. The conditioned media produced by these cells after stimulation with lipopolysaccharide or Candida albicans supports the production and differentiation of granulocytes (ie, the conditioned media contains neutrophil promoting activity [NPA]). We now show that the NPA in the G-CSF(-/-)/GM-CSF(-/-) conditioned media requires interleukin-6 (IL6), is abolished by soluble gp130, and can be specifically immunodepleted by an anti-IL6R antibody. NPA effects on bone marrow cells are also mimicked by Hyper-IL6, and the soluble IL6R is present in NPA. These results show that the IL6/sIL6R complex is the major effector of NPA. NPA production by mice defective for both G-CSF and GM-CSF uncovers an alternative pathway to granulocyte production, which is activated after exposure to pathogens.

Polymorphisms in the interleukin-6 receptor gene are associated with bone mineral density and body mass index in Spanish postmenopausal women

OBJECTIVE

Osteoporosis and obesity are complex diseases with a strong genetic component. Bone mineral density (BMD) and body mass index (BMI) linkage studies identified a locus at 1q21-23, where the interleukin-6 receptor (IL6R) gene is located. The IL6R and the gp130 receptors are the mediators of IL6 action. Serum levels of IL6 and sIL6R (the soluble form of IL6R) are higher in several diseases such as osteoporosis or obesity. Variants at IL6R have been associated with BMI and obesity. However, IL6R is an as-yet-unexplored osteoporosis candidate gene.

DESIGN

In the present study we analysed two polymorphisms in the IL6R promoter, -1435 C/T (rs3887104) and -208 G/A (rs4845617), and the Asp358Ala polymorphism (rs8192284), in relation to both BMD and BMI in a cohort of 559 postmenopausal Spanish women.

RESULTS

The promoter polymorphisms, -1435 C/T and -208 G/A were associated with femoral neck (FN) BMD (P=0.011 and P=0.025 respectively). The C-A and T-G promoter haplotypes were also associated with FN BMD. Additionally, the Asp358Ala variant was associated with lumbar spine BMD (P=0.038). Finally, the -208 G/A polymorphism and the C-G and C-A haplotypes were associated with BMI and obesity, where GG was the risk genotype (P=0.033 for BMI; P=0.010 for obesity).

CONCLUSION

These data suggest that variants in the IL6R gene are not only involved in the determination of BMI but also relevant for the determination of BMD. The IL6R gene may belong to the growing list of genes known to be involved in both phenotypes.

Characterization of the Interleukin (IL)-6 Inhibitor IL-6-RFP

Although fusion proteins of the extracellular parts of receptor subunits termed cytokine traps turned out to be promising cytokine inhibitors for anti-cytokine therapies, their mode of action has not been analyzed. We developed a fusion protein consisting of the ligand binding domains of the IL-6 receptor subunits IL-6Ralpha and gp130 that acts as a highly potent IL-6 inhibitor. Gp130 is a shared cytokine receptor also used by the IL-6-related cytokines oncostatin M and leukemia inhibitory factor. In this study, we have shown that the IL-6 receptor fusion protein (IL-6-RFP) is a specific IL-6 inhibitor that does not block oncostatin M or leukemia inhibitory factor. We characterized the complex of IL-6-RFP and fluorescently labeled IL-6 (YFPIL-6) by blue native PAGE and gel filtration. A 2-fold molar excess of IL-6-RFP over IL-6 was sufficient to entirely bind IL-6 in a complex with IL-6-RFP. As shown by treatment with urea and binding competition experiments, the complex of IL-6 and IL-6-RFP is more stable than the complex of IL-6, soluble IL-6Ralpha, and soluble gp130. By live cell imaging, we have demonstrated that YFP-IL-6 bound to the surface of cells expressing gp130-CFP is removed from the plasma membrane upon the addition of IL-6-RFP. The apparent molecular mass of the IL-6.IL-6-RFP complex determined by blue native PAGE and gel filtration suggests that IL-6 is trapped in a structure analogous to the native hexameric IL-6 receptor complex. Thus, fusion of the ligand binding domains of heteromeric receptors leads to highly specific cytokine inhibitors with superior activity compared with the separate soluble receptors.

Shedding of a soluble form of BMP receptor-IB controls bone cell responses to BMP

Bone morphogenetic proteins (BMP) are members of the transforming growth factor beta (TGF-beta) superfamily and are involved in a wide variety of biological processes, including osteoblast differentiation and bone healing. The activities of the BMP are mediated by signal transduction via three BMP receptors (BMPR-IA, -IB and -II), which are thus essential for the biological actions of the BMP. Although the precise mechanisms which control the BMPR are not yet known, it is possible that post-translational regulation of these cell surface antigens by shedding could modulate their expression and thereby at least partly determine the response of the cells to the BMP. To test this possibility, the present study has examined whether soluble forms of the BMPR are produced by shedding from primary human bone cells in vitro. The results showed that human bone cells expressed both mRNA transcripts and antigens corresponding to BMPR-IA, -IB and -II. Incubation of the cells with phorbol 12-myristate 13-acetate (PMA), a potent inducer of proteolytic shedding, resulted in a pronounced decrease in cell surface expression of all three BMPR and, concurrently, the presence of "soluble" forms of these antigens in culture supernatants. Moreover, PMA treatment significantly reduced the level of BMP-2-induced Smad1/5 phosphorylation, a major early activation step in signal transduction initiated by BMP/BMPR interaction. It is notable that, while treatment of bone cells with interleukin-1beta (IL-1beta) also reduced the level of surface BMPR-IB, this inflammatory cytokine had no effect on BMPR-IA or -II levels, hence only the soluble form of BMPR-IB was detected. Furthermore, in addition to down-regulating BMP-2-induced Smad1/5 phosphorylation, IL-1beta also caused a reduction in the level of BMP-2-induced alkaline phosphatase activity and osteocalcin expression, both closely associated with bone cell differentiation. In conclusion, our study has provided evidence, for the first time, that BMPR can be modulated at the cell surface by the shedding of a soluble form of the antigen, resulting in a markedly diminished response to BMP-2 in vitro.

Bone morphogenetic protein receptors and bone morphogenetic protein signaling are controlled by tumor necrosis factor-α in human bone cells

Bone morphogenetic proteins (BMP) stimulate osteoblast differentiation by signal transduction via three BMP receptors (BMPR-IA, -IB and -II), whereas the inflammatory cytokine tumor necrosis factor-alpha (TNF-alpha) has been shown to suppress osteoblast differentiation. Although the mechanisms which regulate the BMPR are not yet known, it is possible that they may be negatively controlled by TNF-alpha, thereby inhibiting BMP-induced osteoblast differentiation. To test this hypothesis, we have examined the effects of TNF-alpha on BMPR-IA, -IB and -II expression and the functional consequences of this cytokine on BMPR-mediated functions in human bone cells. The results showed that although TNF-alpha down-regulated BMPR-IA and -II transcripts, it increased the level of BMPR-IB mRNA via a MAPK-dependent pathway. In marked contrast, however, TNF-alpha nevertheless caused marked down-regulation of the expression of the BMPR-IB surface antigen specifically. Moreover, the cytokine-induced decrease in BMPR-IB expression was found to be associated with the concurrent presence of a 'soluble' form of this antigen in supernatants of TNF-alpha-treated cultures. Furthermore, the TNF-alpha-induced loss of BMPR-IB was found to ablate BMP-2-stimulated bone cell functions, including phosphorylation of Smad1/5/8, alkaline phosphatase activity and osteocalcin expression. In conclusion, our study has provided evidence, for the first time, that BMPR can be differentially modulated by TNF-alpha at both the post-transcriptional and post-translational levels, with the TNF-alpha-induced shedding of the BMPR-IB antigen associated with a significantly diminished response to BMP-2 in vitro.

Interleukin‐6 receptor expression in contracting human skeletal muscle: regulating role of IL‐6

Contracting muscle fibers produce and release IL-6, and plasma levels of this cytokine are markedly elevated in response to physical exercise. We recently showed autocrine regulation of IL-6 in human skeletal muscle in vivo and hypothesized that this may involve up-regulation of the IL-6 receptor. Therefore, we investigated IL-6 receptor regulation in response to exercise and IL-6 infusion in humans. Furthermore, using IL-6-deficient mice, we investigated the role of IL-6 in the IL-6 receptor response to exercise. Human skeletal muscle biopsies were obtained in relation to: 3 h of bicycle exercise and rest (n=6+5), or recombinant human IL-6 infusion (rhIL-6) or saline infusion (n=6+6). We further obtained skeletal muscle samples from IL-6 knockout (KO) mice and wild-type C57/BL-6 mice in response to a 1-h bout of exercise. In exercising human skeletal muscle, IL-6 receptor mRNA increased sixfold with a peak at 6 h postexercise. Detection of the IL-6 receptor protein by immunohistochemistry revealed a pronounced staining following exercise that was primarily located at the cell membrane of the muscle fibers, whereas muscle gp130 expression and plasma levels of soluble IL-6 receptor were unaffected. Infusion of rhIL-6 to humans had no effect on the mRNA level of the IL-6 receptor, whereas there was an increase at the protein level. IL-6 receptor mRNA increased similarly in muscle of both IL-6 KO mice and wild-type mice in response to exercise. In conclusion, exercise increases IL-6 receptor production in human skeletal muscle. This effect is most prominent 6 h after the end of the exercise bout, suggesting a postexercise-sensitizing mechanism to IL-6 when plasma IL-6 is concomitantly low. Exercise-induced increases in IL-6 receptor mRNA most likely occurs via an IL-6 independent mechanism as shown in IL-6 KO mice and the human rhIL-6 infusion study, whereas IL-6 receptor protein levels are responsive to elevated plasma IL-6 levels.

Fast modulation of heat-activated ionic current by proinflammatory interleukin 6 in rat sensory neurons

The pro-inflammatory cytokine interleukin-6 (IL-6) together with its soluble receptor (sIL-6R) induces and maintains thermal hyperalgesia. It facilitates the heat-induced release of calcitonin gene-related peptide from rat cutaneous nociceptors in vivo and in vitro. Here we report that exposure of nociceptive neurons to the IL-6-sIL-6R complex or the gp130-stimulating designer IL-6-sIL-6R fusion protein Hyper-IL-6 (HIL-6) resulted in a potentiation of heat-activated inward currents (I(heat)) and a shift of activation thresholds towards lower temperatures without affecting intracellular calcium levels. The Janus tyrosine kinase inhibitor AG490, the selective protein kinase C (PKC) inhibitor, bisindolylmaleimide 1 (BIM1), as well as rottlerin, a selective blocker of the PKCdelta isoform, but not the cyclooxygenase inhibitor indomethacin, effectively reduced the effect. Reverse transcription-polymerase chain reaction (RT-PCR) and in situ hybridization revealed expression of mRNA for the signal-transducing beta subunit of the receptor gp130 in neuronal somata, rather than satellite cells in rat dorsal root ganglia. Together, the results suggest that IL-6-sIL-6R acts directly on sensory neurons. It increases their susceptibility to noxious heat via the gp130/Jak/PKCdelta signalling pathway.

Differential regulation of neutrophil-activating chemokines by IL-6 and its soluble receptor isoforms

Interleukin-6 signaling via its soluble receptor (sIL-6R) differentially regulates inflammatory chemokine expression and leukocyte apoptosis to coordinate transition from neutrophil to mononuclear cell infiltration. sIL-6R activities may, however, be influenced in vivo by the occurrence of two sIL-6R isoforms that are released as a consequence of differential mRNA splicing (DS) or proteolytic cleavage (PC) of the cognate IL-6R (termed DS- and PC-sIL-6R). Using human peritoneal mesothelial cells and a murine model of peritoneal inflammation, studies described in this work have compared the ability of both isoforms to regulate neutrophil recruitment. In this respect, DS- and PC-sIL-6R were comparable in their activities; however, these studies emphasized that IL-6 trans signaling differentially controls neutrophil-activating CXC chemokine expression. In vitro, stimulation of mesothelial cells with IL-6 in combination with either DS-sIL-6R or PC-sIL-6R showed no induction of CXC chemokine ligand (CXCL)1 (GRO alpha) and CXCL8 (IL-8), whereas both isoforms enhanced CXCL5 (ENA-78) and CXCL6 (granulocyte chemotactic protein-2) expression. Moreover, when complexed with IL-6, both isoforms specifically inhibited the IL-1 beta-induced secretion of CXCL8. These findings were paralleled in vivo, in which induction of peritoneal inflammation in IL-6-deficient (IL-6(-/-)) mice resulted in enhanced keratinocyte-derived chemokine and macrophage-inflammatory protein-2 (the murine equivalent of CXCL1 and CXCL8) levels, but reduced LPS-induced CXC chemokine (the murine equivalent of CXCL5) expression. Reconstitution of IL-6 signaling in IL-6(-/-) mice with IL-6 and its soluble receptor isoforms corrected this chemokine imbalance and suppressed overall neutrophil infiltration. These data confirm that sIL-6R-mediated signaling primarily limits neutrophil influx; however, induction of CXCL5 and CXCL6 may regulate other neutrophil responses.

A fusion protein of the gp130 and interleukin-6Ralpha ligand-binding domains acts as a potent interleukin-6 inhibitor

Interleukin (IL)-6 is involved in the maintenance and progression of several diseases such as multiple myeloma, rheumatoid arthritis, or osteoporosis. The present work aims at the development of an IL-6 inhibitor for the use in anti-cytokine therapies. The IL-6 receptor is composed of two different subunits, an alpha-subunit (IL-6Ralpha) that binds IL-6 with low affinity and a beta-subunit (gp130) that binds the IL-6.IL-6Ralpha complex with high affinity and as a result triggers intracellular signaling. In its soluble form, gp130 is a natural antagonist that neutralizes IL-6.soluble IL-6Ralpha complexes. It was our strategy to appropriately fuse the two receptor subunit fragments involved in IL-6 receptor complex formation to bind IL-6 with high affinity and to antagonize its effects. The ligand-binding domains of gp130 (D1-D2-D3) and IL-6Ralpha (D2-D3) were connected using three different linkers. The resulting constructs were expressed in stably transfected insect cells and tested for their ability to inhibit IL-6 activity in several in vitro systems. All fusion proteins were strong inhibitors of IL-6 signaling and abrogated IL-6-induced phosphorylation of STAT3, proliferation of transfected Ba/F3 cells, and induction of acute-phase protein synthesis. As intended, the fused receptors were much more effective than the separately expressed soluble receptor proteins. The fusion protein strategy presented here can also be applied to other cytokines that signal via receptors composed of two different subunits to design new potent inhibitors for anti-cytokine therapies.

Clinical manifestations associated with the aberrant expression of the soluble granulocyte-macrophage colony-stimulating factor receptor in patients presenting with haematological malignancies

The receptor for granulocyte-macrophage colony-stimulating factor (GM-CSF) can exist as both transmembrane (tmGMRalpha) and soluble (solGMRalpha) isoforms, and the latter, is a normal constituent of human plasma. We investigated if aberrant solGMRalpha expression occurs in haematopoietic malignancies and whether or not solGMRalpha expression levels correlated with clinical presentation. Compared with the normal population, patients with acute lymphoblastic leukaemia (ALL) had low levels of solGMRalpha whereas clonal disorders of the myeloid lineage demonstrated higher levels of solGMRalpha. Patients with acute myelogenous leukaemia (AML) and high levels of solGMRalpha presented with a distinct clinical picture. These patients were older, predominantly belonged to the M4 and M5 French-American-British (FAB) subtypes, and they had higher white blood cell counts at presentation including myeloid precursors and myeloblasts. They often presented with either unexplained lung infiltrates or hypoxia and lower rates of microbiologically defined infections. Elevated solGMRalpha levels were not associated with decreased relapse-free and overall survival in the AML population. On multivariate analysis, the correlation between elevated solGMRalpha levels and age, M4 and M5 FAB subtypes and decreased numbers of infections persisted. Our study is the first to describe that distinct clinical presentations are associated with aberrant solGMRalpha levels in haematological malignancies.

Granulocyte-macrophage colony-stimulating factor (GM-CSF) and inflammatory stimuli up-regulate secretion of the soluble GM-CSF receptor in human monocytes: evidence for ectodomain shedding of the cell surface GM-CSF receptor alpha subunit

Soluble GM-CSF receptor alpha subunit (sGMRalpha) is a soluble isoform of the GMRalpha that is believed to arise exclusively through alternative splicing of the GMRalpha gene product. The sGMRalpha mRNA is expressed in a variety of tissues, but it is not clear which cells are capable of secreting the protein. We show here that normal human monocytes, but not lymphocytes, constitutively secrete sGMRalpha. Stimulation of monocytes with GM-CSF, LPS, PMA, or A23187 rapidly up-regulates the secretion of sGMRalpha in a dose-dependent manner, demonstrating that secretion is also regulated. To determine whether sGMRalpha arose exclusively through alternative splicing of the GMRalpha gene product, or whether it could also be generated through ectodomain shedding of GMRalpha, we engineered a murine pro-B cell line (Ba/F3) to express exclusively the cDNA for cell surface GMRalpha (Ba/F3.GMRalpha). The Ba/F3.GMRalpha cell line, but not the parental Ba/F3 cell line, constitutively shed a sGMRalpha-like protein that bound specifically to GM-CSF, was equivalent in size to recombinant alternatively spliced sGMRalpha (60 kDa), and was recognized specifically by a mAb raised against the ectodomain of GMRalpha. Furthermore, a broad-spectrum metalloprotease inhibitor (BB94) reduced constitutive and PMA-, A23187-, and LPS-induced secretion of sGMRalpha by monocytes, suggesting that shedding of GMRalpha by monocytes may be mediated in part through the activity of metalloproteases. Taken together, these observations demonstrate that sGMRalpha is constitutively secreted by monocytes, that GM-CSF and inflammatory mediators up-regulate sGMRalpha secretion, and that sGMRalpha arises not only through alternative splicing but also through ectodomain shedding of cell surface GMRalpha.

The role of soluble receptors in cytokine biology: the agonistic properties of the sIL-6R/IL-6 complex

Cytokines perform ever-increasing roles in both, the regulation of general homeostasis and in orchestrating the immune response during disease. To ensure that control of the cytokine network is tightly regulated, nature has developed a series of systems designed for this purpose. In this respect, researchers have placed considerable emphasis on identifying and characterising the regulatory properties of soluble cytokine receptors. These proteins bind their ligands with similar affinities to those of their cognate transmembrane receptors and are effective at prolonging the circulating half-life of cytokines they bind. However, it is the individual capacity of these soluble receptors to act as either antagonists or agonists which has been the principal focus of most research studies. This review provides an overview of the activities of soluble cytokine receptors, but primarily concentrates on those that possess agonistic properties.

Shedding of the interleukin-6 (IL-6) receptor (gp80) determines the ability of IL-6 to induce gp130 phosphorylation in human osteoblasts

Human osteoblasts produce interleukin-6 (IL-6) and respond to IL-6 in the presence of soluble IL-6 receptor (sIL-6R), but the cell surface expression of IL-6R and the mechanism of sIL-6R production are largely unknown. Three different human osteoblast-like cell lines (MG-63, HOS, and SaOS-2) and bone marrow-derived primary human osteoblasts expressed both IL-6R and gp130 as determined by flow cytometry and immunoprecipitation. However, the membrane-bound IL-6R was nonfunctional, as significant tyrosine phosphorylation of gp130 did not occur in the presence of IL-6. Phorbol myristate acetate induced a dramatic increase of both IL-6R shedding (i.e. the production of sIL-6R) and IL-6 release in osteoblast cultures, but the cell surface expression of gp130 remained unchanged. IL-6 complexed with sIL-6R, either exogenously introduced or derived from the nonfunctional cell surface form by shedding, induced rapid tyrosine phosphorylation of gp130. This effect was inhibited by neutralizing antibodies to either sIL-6R or gp130, indicating that the gp130 activation was induced by IL-6/sIL-6R/gp130 interaction. Protein kinase C inhibitors blocked phorbol myristate acetate-induced and spontaneous shedding of IL-6R resulting in the absence of sIL-6R in the culture medium, which in turn also prevented the activation of gp130. In conclusion, human osteoblasts express cell surface IL-6R, which is unable to transmit IL-6-induced signals until it is shed into its soluble form. This unique mechanism provides the flexibility for osteoblasts to control their own responsiveness to IL-6 via the activation of an IL-6R sheddase, resulting in an immediate production of functionally active osteoblast-derived sIL-6R.

Characterisation of Neuropeptide Y Receptor Subtypes by Synthetic NPY Analogues and by Anti-receptor Antibodies

Neuropeptide Y (NPY), a 36-mer neuromodulator, binds to the receptors Y₁, Y₂, Y₄ and Y₅ with nanomolar affinity. They all belong to the rhodopsin-like G-protein coupled, seven transmembrane helix spanning receptors. In this study, Ala-substituted and centrally truncated NPY analogues were compared with respect to affinity to the Y-receptors. Furthermore, antibodies against the second (E2) and the third (E3) extracellular loop of NPY Y₁-, Y₂- and Y₅-receptor subtypes were raised and affinity to intact cells was tested by immunofluorescence assays. Both methods were applied in order to receive subtype selective tools and to characterise ligand binding. The analogues [A¹³]-pNPY and [A²⁷]-pNPY showed subtype selectivity for the Y₂-receptor. Sera against the E2 loop of the Y₁-receptor and against the E2 loop of the Y₂-receptor were subtype selective. Two antibodies against the Y₅ E2 and E3 loop recognised the Y₅- and Y₂-receptor subtypes. In combination, these sera are able to distinguish between the Y₁-, Y₂-, and Y₅-receptor subtypes. The analogues and antibodies represent valuable tools to distinguish NPY receptors on membranes and intact cells.

The soluble interleukin 6 receptor: mechanisms of production and implications in disease

Interleukin 6 (IL-6) performs a prominent role during disease and has been described as both a pro- and anti-inflammatory cytokine. A key feature in the regulation of IL-6 responses has been the identification of a soluble interleukin 6 receptor (sIL-6R), which forms a ligand-receptor complex with IL-6 that is capable of stimulating a variety of cellular responses including proliferation, differentiation and activation of inflammatory processes. Elevated sIL-6R levels have been documented in numerous clinical conditions indicating that its production is coordinated as part of a disease response. Thus, sIL-6R has the potential to regulate both local and systemic IL-6-mediated events. This review will outline the central role of sIL-6R in the coordination of IL-6 responses. Details relating to the mechanisms of sIL-6R production will be provided, while the potential significance of sIL-6R during the development of clinical conditions will be emphasized. We want to convey, therefore, that when thinking about the inflammatory capability of IL-6, it is essential to consider not only the action of IL-6 itself, but also the effect sIL-6R may have on cellular processes.

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

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

Molecular characterization of the human neuropeptide Y Y2-receptor

Five neuropeptide Y receptors, the Y1-, Y2-, Y4-, Y5- and y6-subtypes, have been cloned, which belong to the rhodopsin-like G-protein-coupled, 7-transmembrane helix-spanning receptors and bind the 36-mer neuromodulator NPY (neuropeptide Y) with nanomolar affinity. In this study, the Y2-receptor subtype expressed in a human neuroblastoma cell line (SMS-KAN) and in transfected Chinese hamster ovary cells (CHO-hY2) was characterized on the protein level by using photoaffinity labeling and antireceptor antibodies. Two photoactivatable analogues of NPY were synthesized, in which a Tyr residue was substituted by the photoreactive amino acid 4-(3-trifluoromethyl)-3H-diazirin-3-ylphenylalanine ((Tmd)Phe), [Nalpha-biotinyl-Ahx2,(Tmd)Phe36]NPY (Tmd36), and the Y2-receptor subtype selective [Nalpha-biotinyl-Ahx2,Ahx5-24,(Tmd)Phe27]N PY (Tmd27). Both analogues were labeled with [3H]succinimidyl-propionate at Lys4 and bind to the Y2-receptor with affinity similar to that of the native ligand. A synthetic fragment of the second (E2) extracellular loop was used to generate subtype selective antireceptor antibodies against the Y2-receptor. Photoaffinity labeling of the receptor followed by SDS-PAGE and detection of bound radioactivity and SDS-PAGE of solubilized receptors and subsequent Western blotting revealed the same molecular masses. Two proteins correspondingly have been detected for each cell line with molecular masses of 58 +/- 4 and 50 +/- 4 kDa, respectively.

Evolutionary conservation of circulating soluble low density lipoprotein receptor-related protein-like ("LRP-like") molecules

Low density lipoprotein receptor family members characteristically bind 39-kDa receptor associated protein (RAP). Soluble forms of these receptors have been described in humans including the 515/85-kDa dimeric receptor, low density lipoprotein receptor-related protein (LRP/alpha2MR), which is involved in multiple processes including lipoprotein and protease metabolism. Here we demonstrate evolutionary conservation in the generation of these soluble RAP-binding proteins of high molecular weight, by identifying their presence in mammalian, avian, and reptilian sera as well as in the circulating haemolymph of a mollusc. Sera extracted on immobilized RAP, produced bands at approximately 500 kDa in radiolabeled ligand blots by using the LRP/alpha2MR-specific ligand, Pseudomonas exotoxin A (PEA). These findings suggest that circulating RAP-binding proteins with high molecular weight in vertebrates share features of LRP/alpha2MR (LRP-like molecules). RAP-binding molecules in the mammalian serum extracts were further characterized as LRP/alpha2MR homologues in Western blots by using antibodies against the 515-kDa alpha-chain of LRP/alpha2MR. Western blots of mammalian serum extracts using two monoclonal antibodies recognizing the 85-kDa transmembrane beta-chain suggested that a portion of the beta-chain's ectodomain remains associated with the alpha-chain, but the beta-chain's intracellular carboxy terminus is absent. These results are consistent with evolutionary conservation in the generation, composition, and ligand-binding ability of soluble LRP-like receptors and suggest that their presence is a necessary aspect of the receptor's function.

Comparison of antibodies directed against receptor segments of NPY-receptors

The Y1-, Y2-, Y4- and Y5-receptor, which belong to the rhodopsin-like G-protein coupled, 7 transmembrane helix spanning receptors, bind the 36-mer neuromodulator NPY (neuropeptide Y) with nanomolar affinity. Synthetic fragments of the second (E2) and third (E3) extracellular loop were used to generate subtype selective anti-receptor antibodies against the Y-receptors. As investigated on intact receptors by ELISA and on solubilized receptors by SDS-PAGE and subsequent Western blotting, subtype selectivity was only partly achieved. Nevertheless, selectivity can be obtained by using several antisera in combination. These antibodies represent tools for molecular mass determination, receptor purification by affinity chromatography with antibody-columns and receptor localization studies.

Soluble IL-6 Receptor Potentiates the Antagonistic Activity of Soluble gp130 on IL-6 Responses

Soluble receptors for several cytokines have been detected in body fluids and are believed to modulate the cytokine response by binding the ligand and thereby reducing its bioavailability. In the case of IL-6, the situation is more complex. The receptor consists of two components, including a ligand-binding α-subunit (IL-6R, gp80, or CD126), which in its soluble (s) form (sIL-6R) acts agonistically by making the ligand accessible to the second subunit, the signal transducer gp130 (CD130). Soluble forms of both receptor subunits are present in human blood. Gel filtration of iodinated IL-6 that had been incubated with human serum revealed that IL-6 is partially trapped in IL-6/sIL-6R/sgp130 ternary complexes. sgp130 from human plasma was enriched by immunoaffinity chromatography and identified as a 100-kDa protein. Functionally equivalent rsgp130 was produced in baculovirus-infected insect cells to study its antagonistic potential on four different cell types. It was found that in situations in which cells lacking membrane-bound IL-6R were stimulated with IL-6/sIL-6R complexes, sgp130 was a much more potent antagonist than it was on IL-6R-positive cells stimulated with IL-6 alone. In the latter case, the neutralizing activity of sgp130 could be markedly enhanced by addition of sIL-6R. As a consequence of these findings, sIL-6R of human plasma must be regarded as an antagonistic molecule that enhances the inhibitory activity of sgp130. Furthermore, in combination with sIL-6R, sgp130 is a promising candidate for the development of IL-6 antagonists.

Shedding of the soluble IL-6 receptor is triggered by Ca2+ mobilization, while basal release is predominantly the product of differential mRNA splicing in THP-1 cells

The soluble IL-6 receptor (sIL-6R) is generated through either proteolytic shedding of the cognate receptor (PC-sIL-6R), or released as the product of differential mRNA splicing (DS-sIL-6R). Using monocytic THP-1 cells, we demonstrate that both mechanisms are independently regulated, and that each process contributes to sIL-6R production. Shedding of the IL-6R was activated by the Ca2+ ionophore, ionomycin, and inhibited by the TNF-alpha protease inhibitor (TAPI). In contrast, basal sIL-6R release was unaffected by Ca2+ depletion and largely insensitive to TAPI. Moreover, although IL-6R shedding was inactivated by serum starvation, non-stimulated production remained intact. Basal sIL-6R production via differential mRNA splicing was shown through the inhibitory action of brefeldin A and an enzyme-linked immunosorbent assay specific for DS-sIL-6R. Release of this isoform was unaffected by ionomycin or TAPI, indicating that Ca2+ mobilization activates PC-sIL-6R generation, but not DS-sIL-6R. The divergent control of these sIL-6R isoforms indicates that they may independently influence the inflammatory response.

Acute myeloblastic leukaemia cells produce soluble interleukin 6 receptor by a mechanism of alternative splicing

The aim of the present work was to investigate whether acute myeloblastic leukaemia (AML) blast cells express a soluble (s) form of interleukin 6 (IL-6) receptor (R), and if they do, what is the mechanism of production. Eight AML patient cell lines and 25 primary AML blast cell samples were investigated. The cell lines secreted high quantities of sIL-6R into their culture medium when examined by enzyme-linked immunosorbent assay (ELISA). To determine whether sIL-6R is synthesized by a mechanism of alternative splicing, RNA was analysed from all the AML blast cell samples by using reverse transcription polymerase chain reaction. In this method, primer sites flanking the transmembrane domain were utilized and the alternatively spliced IL-6R mRNA was distinguished from the non-spliced transcript form by size. All the cell lines and 64% of the primary blast cell samples expressed the alternatively spliced IL-6R mRNA. To confirm the phenomenon of alternative splicing at protein level, cytoplasmic protein fractions of the cell lines were investigated by using a sensitive adaptation of the Western blot method. All the cell lines expressed two IL-6R proteins sized 80 and 50 kDa and corresponding to the membraneous and soluble forms of IL-6R, respectively. In conclusion, the results obtained at both mRNA and protein levels strongly support alternative splicing as a mechanism of sIL-6R production in AML. Because sIL-6R modulates the effects of IL-6 on target cells, differences in sIL-6R expression levels may partially explain the previously observed diversity in IL-6-induced growth responses in AML

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

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

Distinct Classes of Chaperoned IL-6 in Human Blood: Differential Immunological and Biological Availability

Transport of IL-6 in blood is fundamental to the biology of this cytokine. In the present study, IL-6 transport, immunological reactivity, and biological availability were investigated in blood from melanoma patients subjected to different active specific immunization regimens (an anti-idiotypic mAb immunization protocol (mAb-keyhole limpet hemocyanin (KLH)-Calmette-Guérin bacillus (BCG), an autologous anti-cancer vaccine protocol (AAAP), or both). Sera were subjected to Sephadex G-200 gel filtration chromatography, and the structure and biological activity of IL-6 complexes in the eluate fractions were probed using five IL-6 ELISAs and two bioassays. Sera from patients administered mAb-KLH+BCG followed by AAAP contained three distinct classes of IL-6 eluting at 30, 200, and 450 kDa, each with its characteristic ELISA reactivity and bioactivity: the 30- and 450-kDa complexes were bioactive in the B9 and Hep3B assays, but the 200-kDa complex was not. The 30- and 450-kDa IL-6 complexes were preferentially reactive in the 7IL6/5IL6 ELISA, the 200-kDa IL-6 complexes were preferentially reactive in the 4IL6/5IL6 ELISA, while the three commercial ELISAs (R&D, Endogen, and Genzyme) detected essentially only the 30-kDa IL-6. In contrast, 1) sera from AAAP patients contained biologically active 30- and 450-kDa IL-6 complexes, while 2) sera from mAb-KLH+BCG patients contained 200-kDa IL-6 complexes inactive in ex vivo bioassays. Both the 450- and 200-kDa complexes included soluble IL-6R, with the 200-kDa complexes additionally containing ligand-occupied anti-IL-6 and anti-soluble IL-6R IgG. The data indicate the existence of specific mechanisms that regulate the transport and function of IL-6 in vivo.

Expression of the interleukin 6 receptor in primary renal cell carcinoma

AIMS

Interleukin 6 (IL-6) is expressed in the majority of renal cell carcinomas and has an important role in the proliferation of some renal cell carcinoma cell lines. This action is mediated by two membrane proteins, gp80 (the IL-6 receptor; IL-6R), which binds IL-6, and gp130, which transduces the signal. The soluble form of gp80 (sIL-6R) is able to activate gp130 when complexed to the IL-6 molecule. These considerations prompted an investigation of IL-6R expression in this malignancy. IL-6, C reactive protein (CRP), and sIL-6R were also measured in serum and correlated to clinical and pathological features.

METHODS

Immunostaining was performed on cryostat sections from renal cell carcinoma tumours with M91, an anti-IL-6R monoclonal antibody, using the alkaline phosphatase antialkaline phosphatase technique. The proliferation index was measured using the KI-67 monoclonal antibody. CRP, IL-6, and sIL-6R were measured in serum before nephrectomy, using an immunoenzymatic or immunoradiometric assay.

RESULTS

There were significant differences in survival in patients with tumours larger than 8 cm, metastasis at diagnosis, high nuclear grade tumours, detectable serum concentrations of IL-6 (correlated to CRP serum concentration), more than 4% proliferating cells, and the presence of the IL-6R in situ. Furthermore, the serum IL-6 concentration correlated with tumour size and stage. The mean serum sIL-6R concentration was not significantly different from that observed in 40 normal subjects. Tumour IL-6R expression was present in 10 samples. There was a significant association between the presence of the IL-6 receptor in tumours and tumour stage, nuclear grade, proliferation index, and serum IL-6.

CONCLUSIONS

This study revealed the importance of IL-6/CRP and IL-6R expression in situ as potential new prognostic factors and opens the way to new therapeutic strategies in renal cell carcinoma.

A complex of the soluble interleukin-6 receptor and interleukin-6 is internalized via the signal transducer gp130

In human body fluids a soluble form of the interleukin-6 receptor (sIL-6R) has been found which together with interleukin-6 (IL-6) acts agonistically on cells expressing the signal transducer gp130. The means by which the sIL-6R is removed from the circulation is unknown. Here, we show that a complex of 125I-labelled recombinant sIL-6R and IL-6 is internalized by MDCK cells stably transfected with gp130 and by human hepatoma cells HepG2 that endogenously express the IL-6R and gp130. We further show that most of the internalized sIL-6R is degraded within lysosomes. Our studies suggest that cells expressing gp130 are capable of endocytosing an IL-6/sIL-6R complex, thereby removing both from the circulation.

Oncostatin M differentially regulates tissue inhibitors of metalloproteinases TIMP-1 and TIMP-3 gene expression in human synovial lining cells

Tissue inhibitor of metalloproteinases (TIMP) 1, 2 and 3 are related proteins that can form complexes with all known matrix metalloproteinases (MMPs). They inhibit the action of MMPs on extracellular matrix components. The balance of MMPs and TIMPs is important for tissue remodeling and its disturbance is believed to play a crucial role in pathophysiological processes such as tumor metastasis, destruction of cartilage and fibrosis. Cytokines and growth factors were found to regulate TIMPs and MMPs in a complex manner. In order to better understand the role of TIMPs in inflammatory joint diseases we have studied in vitro the regulation of TIMP-1 and TIMP-3 by inflammatory cytokines in cultured human synovial lining cells. We found that transforming growth factor beta 1 as well as interleukin-1 beta induce gene expression of both TIMP-1 and TIMP-3. In contrast, oncostatin M, an interleukin-6-type cytokine produced by activated T-lymphocytes and monocytes, had a differential effect on TIMP mRNA levels. After oncostatin M treatment, TIMP-1 expression was up-regulated but basal, as well as interleukin-1 beta-induced, TIMP-3 expression was inhibited. Interleukin-6 itself had no effect on synovial lining cells but a complex of interleukin-6 and the soluble interleukin-6 receptor induced activation of signal transducer and activator of transcription (STAT) factors in these cells and regulated TIMP-1 and TIMP-3 expression in a similar fashion as oncostatin M. Since TIMP-3 is matrix-associated whereas TIMP-1 is found in many body fluids, the role of oncostatin M during inflammatory processes might be to promote ECM degradation in the local environment but to prevent it systemically.