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

Immune-vascular mural cell interactions: consequences for immune cell trafficking, cerebral blood flow, and the blood-brain barrier

Brain barriers are crucial sites for cerebral energy supply, waste removal, immune cell migration, and solute exchange, all of which maintain an appropriate environment for neuronal activity. At the capillary level, where the largest area of brain-vascular interface occurs, pericytes adjust cerebral blood flow (CBF) by regulating capillary diameter and maintain the blood-brain barrier (BBB) by suppressing endothelial cell (EC) transcytosis and inducing tight junction expression between ECs. Pericytes also limit the infiltration of circulating leukocytes into the brain where resident microglia confine brain injury and provide the first line of defence against invading pathogens. Brain "waste" is cleared across the BBB into the blood, phagocytosed by microglia and astrocytes, or removed by the flow of cerebrospinal fluid (CSF) through perivascular routes-a process driven by respiratory motion and the pulsation of the heart, arteriolar smooth muscle, and possibly pericytes. "Dirty" CSF exits the brain and is probably drained around olfactory nerve rootlets and via the dural meningeal lymphatic vessels and possibly the skull bone marrow. The brain is widely regarded as an immune-privileged organ because it is accessible to few antigen-primed leukocytes. Leukocytes enter the brain via the meninges, the BBB, and the blood-CSF barrier. Advances in genetic and imaging tools have revealed that neurological diseases significantly alter immune-brain barrier interactions in at least three ways: (1) the brain's immune-privileged status is compromised when pericytes are lost or lymphatic vessels are dysregulated; (2) immune cells release vasoactive molecules to regulate CBF, modulate arteriole stiffness, and can plug and eliminate capillaries which impairs CBF and possibly waste clearance; and (3) immune-vascular interactions can make the BBB leaky via multiple mechanisms, thus aggravating the influx of undesirable substances and cells. Here, we review developments in these three areas and briefly discuss potential therapeutic avenues for restoring brain barrier functions.

Pleiotropic, Unique and Shared Responses Elicited by IL-6 Family Cytokines in Human Vascular Endothelial Cells

Vascular endothelial cells express glycoprotein 130 (gp130), which is utilized as a signaling receptor by cytokines in the interleukin-6 (IL-6) family. Several IL-6 family cytokines can be found in the circulatory system during physiological or pathological conditions, and may influence endothelial function and response. This study evaluated and compared the cellular and molecular responses induced by IL-6 family cytokines in human endothelial cells. A proteomic analysis showed that IL-6 family cytokines induce the release of a range of proteins from endothelial cells, such as C-C motif chemokine ligand 23, hepatocyte growth factor, and IL-6. Pathway analysis indicated that gp130-signaling in endothelial cells regulates several functions related to angiogenesis and immune cell recruitment. The present investigation also disclosed differences and similarities between different IL-6 family cytokines in their ability to induce protein release and regulate gene expression and intracellular signaling, in regards to which oncostatin M showed the most pronounced effect. Further, this study showed that soluble gp130 preferentially blocks trans-signaling-induced responses, but does not affect responses induced by classic signaling. In conclusion, IL-6 family cytokines induce both specific and overlapping molecular responses in endothelial cells, and regulate genes and proteins involved in angiogenesis and immune cell recruitment.

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.

Unravelling the broader complexity of IL-6 involvement in health and disease

The classification of interleukin-6 (IL-6) as a pro-inflammatory cytokine undervalues the biological impact of this cytokine in health and disease. With broad activities affecting the immune system, tissue homeostasis and metabolic processes, IL-6 displays complex biology. The significance of these involvements has become increasingly important in clinical settings where IL-6 is identified as a prominent target for therapy. Here, clinical experience with IL-6 antagonists emphasises the need to understand the context-dependent properties of IL-6 within an inflammatory environment and the anticipated or unexpected consequences of IL-6 blockade. In this review, we will describe the immunobiology of IL-6 and explore the gamut of IL-6 bioactivity affecting the clinical response to biological drugs targeting this cytokine pathway.

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.

Different Soluble Forms of the Interleukin-6 Family Signal Transducer gp130 Fine-tune the Blockade of Interleukin-6 Trans-signaling

Soluble forms of the IL-6 receptor (sIL-6R) bind to the cytokine IL-6 with similar affinity as the membrane-bound IL-6R. IL-6·sIL-6R complexes initiate IL-6 trans-signaling via activation of the ubiquitously expressed membrane-bound β-receptor glycoprotein 130 (gp130). Inhibition of IL-6 trans-signaling has been shown to be favorable in numerous inflammatory diseases. Furthermore, different soluble forms of gp130 (sgp130) exist that, together with the sIL-6R, are thought to form a buffer for IL-6 in the blood. However, a functional role for the different sgp130 forms has not been described to date. Here we demonstrate that the metalloproteases ADAM10 and ADAM17 can produce sgp130 by ectodomain shedding of gp130, even though this mechanism only accounts for a minor proportion of sgp130 in the circulation. We further show that full-length sgp130 and the shorter forms sgp130-rheumatoid arthritis-associated peptide (RAPS) and sgp130-E10 are differentially expressed in a cell type- specific manner. Remarkably, full-length sgp130 is expressed by monocytes, but this expression is completely lost during differentiation into macrophages in vitro Using genetically engineered murine pre-B cells that secrete different forms of sgp130, we found that these secreted sgp130 proteins are able to prevent trans-signaling-driven cell proliferation of the secreting cells, whereas conditioned supernatant from these cells failed to block IL-6 trans-signaling in other cells. Thus, our data suggest that the different sgp130 forms are released from cells into their immediate surroundings and appear to form cell-associated gradients to modulate their own susceptibility for IL-6 trans-signaling.

Cleavage Site Localization Differentially Controls Interleukin-6 Receptor Proteolysis by ADAM10 and ADAM17

Limited proteolysis of the Interleukin-6 Receptor (IL-6R) leads to the release of the IL-6R ectodomain. Binding of the cytokine IL-6 to the soluble IL-6R (sIL-6R) results in an agonistic IL-6/sIL-6R complex, which activates cells via gp130 irrespective of whether the cells express the IL-6R itself. This signaling pathway has been termed trans-signaling and is thought to mainly account for the pro-inflammatory properties of IL-6. A Disintegrin And Metalloprotease 10 (ADAM10) and ADAM17 are the major proteases that cleave the IL-6R. We have previously shown that deletion of a ten amino acid long stretch within the stalk region including the cleavage site prevents ADAM17-mediated cleavage, whereas the receptor retained its full biological activity. In the present study, we show that deletion of a triple serine (3S) motif (Ser-359 to Ser-361) adjacent to the cleavage site is sufficient to prevent IL-6R cleavage by ADAM17, but not ADAM10. We find that the impaired shedding is caused by the reduced distance between the cleavage site and the plasma membrane. Positioning of the cleavage site in greater distance towards the plasma membrane abrogates ADAM17-mediated shedding and reveals a novel cleavage site of ADAM10. Our findings underline functional differences in IL-6R proteolysis by ADAM10 and ADAM17.

Proteolytic Cleavage Governs Interleukin-11 Trans-signaling

Interleukin (IL)-11 has been shown to be a crucial factor for intestinal tumorigenesis, lung carcinomas, and asthma. IL-11 is thought to exclusively mediate its biological functions through cell-type-specific expression of the membrane-bound IL-11 receptor (IL-11R). Here, we show that the metalloprotease ADAM10, but not ADAM17, can release the IL-11R ectodomain. Chimeric proteins of the IL-11R and the IL-6 receptor (IL-6R) revealed that a small juxtamembrane portion is responsible for this substrate specificity of ADAM17. Furthermore, we show that the serine proteases neutrophil elastase and proteinase 3 can also cleave the IL-11R. The resulting soluble IL-11R (sIL-11R) is biologically active and binds IL-11 to activate cells. This IL-11 trans-signaling pathway can be inhibited specifically by the anti-inflammatory therapeutic compound sgp130Fc. In conclusion, proteolysis of the IL-11R represents a molecular switch that controls the IL-11 trans-signaling pathway and widens the number of cells that can be activated by IL-11.

Blockade of IL-6 Trans signaling attenuates pulmonary fibrosis

Idiopathic pulmonary fibrosis (IPF) is a lethal lung disease with progressive fibrosis and death within 2-3 y of diagnosis. IPF incidence and prevalence rates are increasing annually with few effective treatments available. Inhibition of IL-6 results in the attenuation of pulmonary fibrosis in mice. It is unclear whether this is due to blockade of classical signaling, mediated by membrane-bound IL-6Rα, or trans signaling, mediated by soluble IL-6Rα (sIL-6Rα). Our study assessed the role of sIL-6Rα in IPF. We demonstrated elevations of sIL-6Rα in IPF patients and in mice during the onset and progression of fibrosis. We demonstrated that protease-mediated cleavage from lung macrophages was important in production of sIL-6Rα. In vivo neutralization of sIL-6Rα attenuated pulmonary fibrosis in mice as seen by reductions in myofibroblasts, fibronectin, and collagen in the lung. In vitro activation of IL-6 trans signaling enhanced fibroblast proliferation and extracellular matrix protein production, effects relevant in the progression of pulmonary fibrosis. Taken together, these findings demonstrate that the production of sIL-6Rα from macrophages in the diseased lung contributes to IL-6 trans signaling that in turn influences events crucial in pulmonary fibrosis.

Soluble interleukin-6 receptor is elevated during influenza A virus infection and mediates the IL-6 and IL-32 inflammatory cytokine burst

Influenza A virus (IAV) infection is a major worldwide public health problem. However, the factors involved in mediating the inflammatory response to this infection and their relationships remain poorly understood. Here, we show that IAV infection stimulates the expression of the soluble IL-6 receptor (sIL-6R), a multifunctional protein involved in IL-6 signaling. Interestingly, sIL-6R expression upregulated the levels of its own ligand, IL-6 and those of the pro-inflammatory cytokine IL-32. shRNA-mediated knockdown of sIL-6R suppressed IL-6 and IL-32, indicating that this regulation is dependent on sIL-6R during IAV infection. Furthermore, our results demonstrate that IL-32 participates in a negative feedback loop that inhibits sIL-6R while upregulating IL-6 expression during IAV infection. Therefore, we show that sIL-6R is a critical cellular factor involved in the acute inflammatory response to viral infection.

Soluble TNF and IL-6 receptors: indicators of vascular health in women without cardiovascular disease

Cytokine receptor subunits are released from cells in a regulated manner and circulate in soluble forms at concentrations that are orders of magnitude greater than the concentrations of the cytokines themselves. The purpose of this study was to determine if the circulating concentrations of soluble receptor subunits for interleukin-1 beta (IL-1β), tumor necrosis factor alpha (TNFα) and interleukin-6 (IL-6) might serve as early indicators of vascular dysfunction independent of the traditional cardiovascular disease (CVD) risk factors in women. Healthy women, aged 20-50 years (n = 36), were assessed for circulating concentrations of the cytokines IL-1β, IL-6 and TNFα and the soluble cytokine receptor subunits interleukin-1 receptor type I (sIL-1RI), sIL-1RII, sIL-6Rα, glycoprotein 130 (s-gp130), soluble TNF receptor type 1 (sTNFR1), and sTNFR2, along with traditional CVD risk factors. Cytokine receptor subunit expression on mononuclear cells and the release of these subunits in vitro were also determined. Brachial artery flow-mediated dilation (FMD), carotid intima-media thickness (cIMT) and carotid-femoral pulse wave velocity (cfPWV) were assessed by ultrasonography and Doppler probes. Circulating sIL-6Rα correlated negatively with FMD (r = -0.56, p = 0.007) independent of age and other CVD risk factors. Circulating sTNFR1 correlated positively with cfPWV (r = 0.60, p = 0.002). TNFR1 receptor expression on monocytes correlated positively with cIMT (r = 0.51, p = 0.004). Plasma concentrations of IL-1β, IL-6 and TNFα were not significantly associated with FMD, cIMT or cfPWV. These data suggest that the receptors for IL-6 and TNFα, rather than the cytokines themselves, may be better indicators of early vascular changes that are associated with CVD.

Interleukin-6: from basic biology to selective blockade of pro-inflammatory activities

Cytokines receptors exist in membrane bound and soluble form. A soluble form of the human IL-6R is generated by limited proteolysis and alternative splicing. The complex of IL-6 and soluble IL-6R stimulates target cells not stimulated by IL-6 alone, since they do not express the membrane bound IL-6R. We have named this process trans-signaling. Soluble gp130 is the natural inhibitor of IL-6/soluble IL-6R complex responses. Recombinant soluble gp130 protein is a molecular tool to discriminate between gp130 responses via membrane bound and soluble IL-6R responses. Neutralizing monoclonal antibodies for global blockade of IL-6 signaling and the sgp130Fc protein for selective blockade of IL-6 trans-signaling have been used in several animal models of human diseases. Using the sgp130Fc protein or sgp130Fc transgenic mice we demonstrate in models of inflammatory bowel disease, peritonitis, rheumatoid arthritis, atherosclerosis pancreatitis, colon cancer, ovarian cancer and pancreatic cancer, that IL-6 trans-signaling via the soluble IL-6R is the crucial step in the development and the progression of the disease. Therefore, sgp130Fc is a novel therapeutic agent for the treatment of chronic inflammatory diseases and cancer and it undergoes phase I clinical trials as an anti-inflammatory drug since June 2013.

Soluble interleukin-6 receptor-mediated innate immune response to DNA and RNA viruses

The interleukin-6 (IL-6) receptor, which exists as membrane-bound and soluble forms, plays critical roles in the immune response. The soluble IL-6 receptor (sIL6R) has been identified as a potential therapeutic target for preventing coronary heart disease. However, little is known about the role of this receptor during viral infection. In this study, we show that sIL6R, but not IL-6, is induced by viral infection via the cyclooxygenase-2 pathway. Interestingly, sIL6R, but not IL-6, exhibited extensive antiviral activity against DNA and RNA viruses, including hepatitis B virus, influenza virus, human enterovirus 71, and vesicular stomatitis virus. No synergistic effects on antiviral action were observed by combining sIL6R and IL-6. Furthermore, sIL6R mediated antiviral action via the p28 pathway and induced alpha interferon (IFN-α) by promoting the nuclear translocation of IFN regulatory factor 3 (IRF3) and NF-κB, which led to the activation of downstream IFN effectors, including 2',5'-oligoadenylate synthetase (OAS), double-stranded RNA-dependent protein kinase (PKR), and myxovirus resistance protein (Mx). Thus, our results demonstrate that sIL6R, but not IL-6, plays an important role in the host antiviral response.

Therapeutic strategies for the clinical blockade of IL-6/gp130 signaling

The successful treatment of certain autoimmune conditions with the humanized anti-IL-6 receptor (IL-6R) antibody tocilizumab has emphasized the clinical importance of cytokines that signal through the β-receptor subunit glycoprotein 130 (gp130). In this Review, we explore how gp130 signaling controls disease progression and examine why IL-6 has a special role among these cytokines as an inflammatory regulator. Attention will be given to the role of the soluble IL-6R, and we will provide a perspective into the clinical blockade of IL-6 activity in autoimmunity, inflammation, and cancer.

Species specificity of ADAM10 and ADAM17 proteins in interleukin-6 (IL-6) trans-signaling and novel role of ADAM10 in inducible IL-6 receptor shedding

Hypomorphic ADAM17(ex/ex) mice showed defects in mucosal regeneration due to inefficient enhanced GFR shedding. ADAM17 is the main sheddase of interleukin-6 receptor (IL-6R) to induce IL-6 trans-signaling. However, serum levels of soluble murine IL-6R were not reduced in ADAM17(ex/ex) mice, and murine ADAM17 was not the major sheddase of murine IL-6R. Shedding of murine IL-6R by murine ADAM17 was rescued in chimeric murine IL-6R proteins containing any extracellular domain but not the transmembrane and intracellular domain of human IL-6R. Apoptosis is a physiological stimulus of ADAM17-mediated shedding of human IL-6R. Even though apoptosis induced IL-6R shedding in mice, the responsible protease was identified as ADAM10. ADAM10 also was identified as protease responsible for ionomycin-induced shedding of murine and human IL-6R. However, in ADAM10-deficient murine embryonic fibroblasts, compensatory shedding of human IL-6R was mediated by ADAM17, but loss of ADAM10-mediated shedding of murine IL-6R was compensated by an as-yet-unidentified protease. Finally, we identified physiological purinergic P2X7 receptor stimulation as a novel inducer of murine and human IL-6R shedding solely mediated by ADAM10. In conclusion, we describe an unexpected species specificity of ADAM10 and ADAM17 and identified ADAM10 as novel inducible sheddase of IL-6R in mice and humans, which might have consequences for the interpretation of phenotypes from ADAM17- and ADAM10-deficient mice.

Expression of soluble interleukin-6 receptor in malignant ovarian tissue

OBJECTIVE

The objective of the study was to investigate interleukin-6 receptor (IL6R) isoforms and sheddases in the ovarian tumor microenvironment.

STUDY DESIGN

Expression of IL6R and sheddases was measured in tissue samples of papillary serous ovarian carcinomas and benign ovaries by real-time polymerase chain reaction and immunohistochemistry. Murine xenograft samples were tested by enzyme-linked immunosorbent assay to discriminate and evaluate tumor and host contributions of IL6R.

RESULTS

IL6R expression was increased in malignant ovarian tumors and localized to epithelial cells. Expression of a soluble splice variant of IL6R was increased in malignant tumors, as were the sheddases for the full-length isoform. An in vivo xenograft model showed that host IL6R expression is also increased and regulated by tumor-associated inflammation.

CONCLUSION

IL6R is overexpressed in epithelial ovarian malignancies because of increases in a soluble IL6R variant, in the sheddases for full-length IL6R and host IL6R expression. Soluble IL6R may be an efficacious target for reducing IL6-mediated ovarian tumor progression.

High habitual dietary alpha-linolenic acid intake is associated with decreased plasma soluble interleukin-6 receptor concentrations in male twins

BACKGROUND

alpha-Linolenic acid (ALA) is associated with a low risk of cardiovascular disease; however, the underlying mechanism is not completely known.

OBJECTIVE

The objective was to examine whether habitual dietary ALA intake is associated with plasma concentrations of inflammatory biomarkers after control for shared genetic and common environmental factors.

DESIGN

We cross-sectionally studied 353 middle-aged male twins. Habitual diet was assessed with the Willett food-frequency questionnaire. Fasting plasma concentrations of interleukin-6 (IL-6) and its soluble receptor (sIL-6R), high-sensitivity C-reactive protein (hsCRP), and tumor necrosis factor-alpha (TNF-alpha) were measured. Linear mixed-effect regression analysis was used to partition the overall association into within- and between-pair associations.

RESULTS

A 1-g increment in habitual dietary ALA intake was associated with 11.0% lower concentrations of sIL-6R (P = 0.004) but not of IL-6 (P = 0.31), TNF-alpha (P = 0.16), or hsCRP (P = 0.36) after adjustment for energy intake, nutritional factors, known cardiovascular disease risk factors, and medications. After further control for shared genetic and common environmental factors by comparison of brothers within a twin pair, a twin with a 1-g higher ALA intake was likely to have 10.9% (95% CI: 3.7%, 17.6%; P = 0.004) lower sIL-6R concentrations than his co-twin with a low intake, whereas ALA intake was not significantly associated with plasma concentrations of IL-6, TNF-alpha, or hsCRP. These results were validated by using 1000 bootstrap samples.

CONCLUSIONS

Habitual dietary ALA intake is inversely associated with plasma sIL-6R concentrations independent of shared genetic and common environmental influences. Lowering sIL-6R may be a mechanism underlying the cardioprotective properties of habitual dietary ALA. This study was registered at clinicaltrials.gov as NCT00017836.

The response of plasma interleukin-6 and its soluble receptors to exercise in the cold in humans

In this study, we wished to determine whether the changes in metabolism observed during exercise in the cold are associated with changes in interleukin-6 (IL-6) and/or its soluble receptors. Eight healthy male participants performed 1 h of cycling exercise at 70% VO2max in a control (20 degrees C) and cold (0 degrees C) environment. Plasma concentrations of IL-6, soluble IL-6 receptor (sIL-6R), and sgp130 were measured before exercise, at 30 and 60 min of exercise, and 60 min after exercise. Substrate oxidation was estimated through measures of pulmonary gas exchange recorded between 50 and 55 min of cycling. Exercise in the cold resulted in an increase (P < 0.05) in carbohydrate oxidation (mean 2.58 g.min(-1), s = 0.49 at 20 degrees C vs. 2.85 g.min(-1), s = 0.58 at 0 degrees C) and a decrease (P < 0.05) in fat oxidation (0.55 g.min(-1), s = 0.17 at 20 degrees C vs. 0.38 g.min(-1), s = 0.16 at 0 degrees C) compared with the control trial. Interleukin-6 concentrations were elevated (P < 0.05) after 60 min of exercise in both the cold and control trials, with no differences between trials at any instant. Neither sIL-6R nor sgp130 was affected by exercise or the environment. The alterations in carbohydrate and fat utilization during 1 h of exercise in the cold are not paralleled by changes in plasma concentrations of IL-6 or its soluble receptors.

Cutting edge: soluble IL-6R is produced by IL-6R ectodomain shedding in activated CD4 T cells

IL-6 trans-signaling via the soluble IL-6R (sIL-6R) plays an important role in the progression of several autoimmune diseases and cancer by providing IL-6-responsiveness to cells lacking IL-6R. However, the potential sources of sIL-6R are less understood. In this study we show that sIL-6R is produced by both naive and memory CD4 T cells upon TCR activation. The production of sIL-6R by activated CD4 T cells is mediated by shedding of the membrane-bound IL-6R, and this process correlates with the expression of the metalloproteinase ADAM17 in these cells. In contrast to CD4 T cells, CD8 T cells do not express ADAM17 and their production of sIL-6R is negligible. Thus, during an immune response CD4 T cells are an important source of sIL-6R. Production of sIL-6R by autoreactive CD4 T cells may contribute to their role in the development of autoimmune disease by conferring IL-6-responsiveness to cells lacking IL-6R such as synoviocytes.

Response of plasma IL-6 and its soluble receptors during submaximal exercise to fatigue in sedentary middle-aged men

The pleiotropic cytokine interleukin-6 (IL-6) has been demonstrated to increase during exercise. Little is known regarding the response of the soluble IL-6 receptors (sIL-6R and sgp130) during such exercise. The aim of the current study was to investigate the response of plasma IL-6, sIL-6R and sgp130 during fatiguing submaximal exercise in humans. Twelve participants underwent an incremental exercise test to exhaustion and one week later performed a submaximal exercise bout (96 +/- 6% lactate threshold) to volitional exhaustion. Blood samples taken at rest and immediately post exercise were analyzed for IL-6, sIL-6R and sgp130. IL-6 increased (P < 0.01) by 8.4 +/- 8.9 pg ml(-1) (75.7%) during the exercise period. sIL-6R and sgp130 also increased (P < 0.05) by 2.7 +/- 3.9 ng ml(-1) (9.6%) and 37.7 +/- 55.6 ng ml(-1) (9.6%), respectively. The current study is the first investigation to demonstrate that alongside IL-6, acute exercise stress results in an increase in both sIL-6R and sgp130.

Changes in Expression of Interleukin-6 Receptors in Granulosa Cells During Follicular Atresia in Pig Ovaries

More than 99% of follicles undergo a degenerative process known as "atresia" in mammalian ovaries, and only a few follicles ovulate during follicular growth and development. Follicular selection predominantly depends on granulosa cell apoptosis. To reveal the molecular mechanisms of selective follicular atresia, we examined the changes in the levels of interleukin-6 (IL-6) receptors expressed in the granulosa cells of pig ovaries. The levels of IL-6 receptor (IL-6R)-alpha mRNA and protein in granulosa cells were quantified by reverse transcription-polymerase chain reaction (RT-PCR) and Western blotting, respectively. IL-6R alpha mRNA and protein were highly expressed in the granulosa cells of progressed atretic follicles. Enzyme-linked immunosorbent assay showed that the expression of IL-6 soluble receptor (IL-6sR) protein in follicular fluid decreased during atresia. Moreover, we isolated porcine cDNA encoding an IL-6 signal transducer, gp130. Porcine gp130 (2,754 bp and 917 amino acids) was identified from a cDNA library prepared using follicular granulosa cells of pig ovaries. Porcine gp130 was highly homologous with human and murine gp130. RT-PCR analysis revealed that the level of gp130 mRNA also decreased during atresia. We presume that IL-6sR and gp130, but not IL-6R alpha, play important roles in regulation of granulosa cell survival.

Human T-cell leukemia virus type-I Tax induces expression of interleukin-6 receptor (IL-6R): Shedding of soluble IL-6R and activation of STAT3 signaling

Human T-cell leukemia virus type-I (HTLV-I) encodes for the viral protein Tax, which is known to significantly disrupt transcriptional control of cytokines, cytokine receptors and other immuno-modulatory proteins in T cells. Specific dysregulation of these factors can alter the course and pathogenesis of infection. Soluble interleukin-6 receptor (sIL-6R) was shown to circulate at elevated levels in HTLV-I-infected patients, and high expressions of IL-6R and sIL-6R by HTLV-I-infected T cells were clinically and experimentally associated with Tax activity. To examine roles of Tax in expression of the IL-6R gene, the JPX-9 cell line was used, which is derived from Jurkat cell line expressing Tax cDNA. Over-expression of Tax enhanced IL-6R expression but not in Tax mutant JPX-9/M cell line. The clinical relevance of these observations was further demonstrated by ELISA using sera obtained from HTLV-I-infected patients. Our results revealed that sIL-6R levels were apparently elevated in HAM/TSP patients who were expressing Tax in their cells, while ATL patients' cells barely expressed Tax. HTLV-I-infected T-cell lines stimulated by IL-6/sIL-6R showed gp130-mediated STAT3 activity. IL-6/sIL-6R enhanced proliferation of HTLV-I-infected T cells in association with activation of STAT3. Consequently, Tax-mediated regulations of IL-6R and sIL-6R observed in HTLV-I-associated disorders may contribute to proliferation of HTLV-I-infected T cells through activation of inducible STAT3, and ultimately affect malignant growth and transformation of T cells by HTLV-I.

A role for exosomes in the constitutive and stimulus-induced ectodomain cleavage of L1 and CD44

Ectodomain shedding is a proteolytic mechanism by which transmembrane molecules are converted into a soluble form. Cleavage is mediated by metalloproteases and proceeds in a constitutive or inducible fashion. Although believed to be a cell-surface event, there is increasing evidence that cleavage can take place in intracellular compartments. However, it is unknown how cleaved soluble molecules get access to the extracellular space. By analysing L1 (CD171) and CD44 in ovarian carcinoma cells, we show in the present paper that the cleavage induced by ionomycin, APMA (4-aminophenylmercuric acetate) or MCD (methyl-beta-cyclodextrin) is initiated in an endosomal compartment that is subsequently released in the form of exosomes. Calcium influx augmented the release of exosomes containing functionally active forms of ADAM10 (a disintegrin and metalloprotease 10) and ADAM17 [TACE (tumour necrosis factor a-converting enzyme)] as well as CD44 and L1 cytoplasmic cleavage fragments. Cleavage could also proceed in released exosomes, but only depletion of ADAM10 by small interfering RNA blocked cleavage under constitutive and induced conditions. In contrast, cleavage of L1 in response to PMA occurred at the cell surface and was mediated by ADAM17. We conclude that different ADAMs are involved in distinct cellular compartments and that ADAM10 is responsible for shedding in vesicles. Our findings open up the possibility that exosomes serve as a platform for ectodomain shedding and as a vehicle for the cellular export of soluble molecules.

Bacterial induction of TNF-alpha converting enzyme expression and IL-6 receptor alpha shedding regulates airway inflammatory signaling

Airway epithelial cells have a major role in initiating inflammation in response to bacterial pathogens. Through the immediate induction of CXCL8 and cytokine expression, polymorphonuclear cells are mobilized and activated to eradicate the infecting organisms. However, the influx of polymorphonuclear cells and the effects of their toxic exoproducts impede respiratory function. We postulated that respiratory epithelial cells must also participate in the regulation of their own proinflammatory signaling. Both Staphylococcus aureus and Pseudomonas aeruginosa were found to potently activate IL-6 expression immediately upon contact with epithelial cells, and by 1 h induced TNF-alpha converting enzyme (TACE) transcription. By 4 h of bacterial exposure, TACE colocalized with IL-6Ralpha on the apical surface of airway cells, and by 24 h, soluble IL-6Ralpha accumulated in the cell culture supernatant. Epithelial IL-6 and soluble IL-6Ralpha were shown to participate in trans-signaling, interacting with membrane-associated gp130 to activate CCL-2 expression and inhibit additional CXCL8 production. Thus, bacteria are physiological activators of TACE expression, which provides a mechanism to regulate inflammatory signaling that is initiated by airway epithelial cells.

Alternative splicing generates putative soluble CD83 proteins that inhibit T cell proliferation

CD83 is expressed on mature dendritic cells and activated lymphocytes and has been implicated to play an important role during T cell development in the thymus. In contrast, not much is known about the function of CD83 in the periphery. Soluble forms of CD83 have been detected in the serum, but neither the function nor the mechanism of how these soluble forms of CD83 are generated are fully understood. In this study, we report the identification of four different transcripts of CD83 in unstimulated PBMCs. Sequence analysis demonstrated that the longest form codes for transmembrane CD83 (CD83-TM), whereas the smaller transcripts are splice variants of full-length CD83, coding for putative soluble CD83 proteins. Stimulation of PBMCs with PHA, TNF-alpha, or LPS leads to the up-regulation of the full-length CD83 transcript and to a strong down-regulation of two of the three smaller transcripts. The smallest CD83 splice product can be translated efficiently into protein, and recombinant soluble CD83 shows a strong inhibitory effect on T cell proliferation in MLRs. Our results suggest that the constitutive production of soluble forms of CD83 under steady-state conditions may have an important function in regulating immune homeostasis.

In vivo modulation of soluble "antagonistic" IL-6 receptor synthesis and release in ESRD

Soluble gp130 (sgp130) is a soluble circulating receptor of IL-6 with "antagonistic" biologic activity. It is generated independently by either shedding of the extracellular domain of membrane gp130 or alternative mRNA splicing. This study was addressed to clarify the mechanisms underlying sgp130 synthesis and release in patients who undergo regular dialysis treatment (RDT) using dialytic membranes with different biocompatibility. Two groups of RDT patients were enrolled: 11 patients who were treated with cellulosic membranes (C) and 10 patients who were treated with synthetic membranes (S). Ten healthy subjects constituted the control group. Serum samples and peripheral blood mononuclear cells (PBMC) were harvested in all groups (before dialysis in RDT patients). PBMC were cultured for 24 h in the absence or presence of LPS. The serum levels of sgp130 were significantly higher in C group than in control and S patients (C, 603.1 +/- 89.9; control, 396 +/- 49.5; S, 423.4 +/- 27.7 ng/ml; P < 0.01). PBMC from C patients, in the absence of any mitogenic stimulation, released a significantly greater amount of sgp130 as compared with S and control groups (C, 532.6 +/- 161.2; S, 332.4 +/- 148.6; control, 341.4 +/- 125.4 pg/ml; P < 0.01). The sgp130 release was positively correlated with the release of both IL-6 (r = 0.336, P < 0.05) and sIL-6R receptor (r = 0.324, P < 0.05). A significantly higher gp130 gene expression was also observed in unstimulated PBMC from C patients when compared with control and S groups. It is interesting that the expression of the 85-bp exon characteristic of the alternative splicing mRNA for sgp130 was low in all groups. Finally, confocal microscopy analysis showed an increased expression of gp130 on cell surface in unstimulated PBMC from C patients as compared with control and S groups. Our results demonstrate that in patients on RDT with C membranes, the synthesis and release of sgp130 "antagonistic" receptor is significantly increased. This release is seemingly due to a shedding of membrane-bound gp130 receptor. The increased sgp130 release may partially counteract the inflammatory effects caused by IL-6.

Interleukin-6 receptor shedding is enhanced by interleukin-1? and tumor necrosis factor ? and is partially mediated by tumor necrosis factor ?-converting enzyme in osteoblast-like cells

OBJECTIVE

Interleukin-6 (IL-6) and soluble IL-6 receptor (sIL-6R) activation of gp130 represents an alternative pathway for osteoclast development in inflammatory conditions. The goal of the present study was to investigate changes in sIL-6R levels in response to the inflammatory cytokines IL-1beta and tumor necrosis factor alpha (TNFalpha) and to determine the role of TNFalpha-converting enzyme (TACE) in this process.

METHODS

Levels of sIL-6R in the culture media of MG63 and SAOS-2 osteoblast-like cell lines after exposure to various agents were determined by immunoassay. TACE protein levels were measured by Western immunoblotting. Cells were transfected with small interfering RNA (siRNA) or with an expression plasmid for IL-6R and TACE to determine the potential involvement of TACE in IL-6R shedding.

RESULTS

IL-1beta and TNFalpha increased the levels of sIL-6R in the culture media of MG63 osteoblast-like cells. This effect was not influenced by cycloheximide or 5,6-dichlorobenzimidazole riboside but was markedly inhibited by the calcium chelator EGTA and by the TACE and matrix metalloproteinase inhibitor hydroxamate (Ru36156). IL-1beta and TNFalpha had no influence on the alternatively spliced form of IL-6R RNA. Levels of sIL-6R were reduced when MG63 cells were transiently transfected with TACE siRNA. Transfection of SAOS-2 cells with expression plasmids for IL-6R and TACE produced a dose-dependent increase in sIL-6R levels.

CONCLUSION

IL-1beta- and TNFalpha-mediated induction of IL-6R shedding in osteoblast-like cells is at least partly dependent on TACE activation.

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.

Natural, Proteolytic Release of a Soluble Form of Human IL-15 Receptor α-Chain That Behaves as a Specific, High Affinity IL-15 Antagonist

IL-15 and IL-2 are two structurally and functionally related cytokines whose high affinity receptors share the IL-2R beta-chain and gamma-chain in association with IL-15R alpha-chain (IL-15R alpha) or IL-2R alpha-chain, respectively. Whereas IL-2 action seems restricted to the adaptative T cells, IL-15 appears to be crucial for the function of the innate immune responses, and the pleiotropic expression of IL-15 and IL-15R alpha hints at a much broader role for the IL-15 system in multiple cell types and tissues. In this report, using a highly sensitive radioimmunoassay, we show the existence of a soluble form of human IL-15R alpha (sIL-15R alpha) that arises from proteolytic shedding of the membrane-anchored receptor. This soluble receptor is spontaneously released from IL-15R alpha-expressing human cell lines as well as from IL-15R alpha transfected COS-7 cells. This release is strongly induced by PMA and ionomycin, and to a lesser extent by IL-1 beta and TNF-alpha. The size of sIL-15R alpha (42 kDa), together with the analysis of deletion mutants in the ectodomain of IL-15R alpha, indicates the existence of cleavage sites that are proximal to the plasma membrane. Whereas shedding induced by PMA was abrogated by the synthetic matrix metalloproteinases inhibitor GM6001, the spontaneous shedding was not, indicating the occurrence of at least two distinct proteolytic mechanisms. The sIL-15R alpha displayed high affinity for IL-15 and behaved as a potent and specific inhibitor of IL-15 binding to the membrane receptor, and of IL-15-induced cell proliferation (IC(50) in the range from 3 to 20 pM). These results suggest that IL-15R alpha shedding may play important immunoregulatory functions.

Shedding of the type II IL-1 decoy receptor requires a multifunctional aminopeptidase, aminopeptidase regulator of TNF receptor type 1 shedding

Proteolytic cleavage of the extracellular domain of the type II IL-1 decoy receptor (IL-1RII) generates soluble IL-1-binding proteins that prevent excessive bioactivity by binding free IL-1. In this study we report that an aminopeptidase, aminopeptidase regulator of TNFR1 shedding (ARTS-1), is required for IL-1RII shedding. Coimmunoprecipitation experiments demonstrate an association between endogenous membrane-associated ARTS-1 and a 47-kDa IL-1RII, consistent with ectodomain cleavage of the membrane-bound receptor. A direct correlation exists between ARTS-1 protein expression and IL-1RII shedding, as cell lines overexpressing ARTS-1 have increased IL-1RII shedding and decreased membrane-associated IL-1RII. Basal IL-1RII shedding is absent from ARTS-1 knockout cell lines, demonstrating that ARTS-1 is required for constitutive IL-1RII shedding. Similarly, PMA-mediated IL-1RII shedding is almost entirely ARTS-1-dependent. ARTS-1 expression also enhances ionomycin-induced IL-1RII shedding. ARTS-1 did not alter levels of membrane-associated IL-1RI or IL-1R antagonist release from ARTS-1 cell lines, which suggests that the ability of ARTS-1 to promote shedding of IL-1R family members may be specific for IL-1RII. Further, increased IL-1RII shedding by ARTS-1-overexpressing cells attenuates the biological activity of IL-1beta. We conclude that the ability of ARTS-1 to enhance IL-1RII shedding represents a new mechanism by which IL-1-induced cellular events can be modulated. As ARTS-1 also promotes the shedding of the structurally unrelated 55-kDa, type I TNF receptor and the IL-6R, we propose that ARTS-1 may play an important role in regulating innate immune and inflammatory responses by increasing cytokine receptor shedding.

The soluble form of CD83 is present at elevated levels in a number of hematological malignancies

Recombinant forms of soluble CD83 (sCD83) inhibit anti-tumor responses. In this analysis of circulating sCD83 levels we report that although >95% of acute myeloid leukemia (AML) and multiple myeloma (MM) patients have normal or only weakly elevated sCD83 levels, 20% of chronic lymphocytic leukemia (CLL) and 5/7 mantle cell lymphoma (MCL) patients have significantly elevated levels (>1 ng/ml). Isolated CLL cells both weakly expressed membrane CD83 (mCD83), and released sCD83 during in vitro culture. We conclude that malignant cells are a potential source of sCD83 and that it may have functional and/or prognostic significance in hematological malignancies, particularly CLL and MCL.

An aminopeptidase, ARTS-1, is required for interleukin-6 receptor shedding

Aminopeptidase regulator of TNFR1 shedding (ARTS-1) binds to the type I tumor necrosis factor receptor (TNFR1) and promotes receptor shedding. Because hydroxamic acid-based metalloprotease inhibitors prevent shedding of both TNFR1 and the interleukin-6 receptor (IL-6Ralpha), we hypothesized that ARTS-1 might also regulate shedding of IL-6Ralpha, a member of the type I cytokine receptor superfamily that is structurally different from TNFR1. Reciprocal co-immunoprecipitation experiments identified that membrane-associated ARTS-1 directly binds to a 55-kDa IL-6Ralpha, a size consistent with soluble IL-6Ralpha generated by ectodomain cleavage of the membrane-bound receptor. Furthermore, ARTS-1 promoted IL-6Ralpha shedding, as demonstrated by a direct correlation between increased membrane-associated ARTS-1 protein, increased IL-6Ralpha shedding, and decreased membrane-associated IL-6Ralpha in cell lines overexpressing ARTS-1. The absence of basal IL-6Ralpha shedding from arts-1 knock-out cells identified that ARTS-1 was required for constitutive IL-6Ralpha shedding. Furthermore, the mechanism of constitutive IL-6Ralpha shedding requires ARTS-1 catalytic activity. Thus, ARTS-1 promotes the shedding of two cytokine receptor superfamilies, the type I cytokine receptor superfamily (IL-6Ralpha) and the TNF receptor superfamily (TNFR1). We propose that ARTS-1 is a multifunctional aminopeptidase that may modulate inflammatory events by promoting IL-6Ralpha and TNFR1 shedding.

Multiple metalloproteinases process protransforming growth factor-alpha (proTGF-alpha)

Shedding of TNF-alpha requires a single cleavage event, whereas the ectodomain of proTGF-alpha is cleaved at N-proximal (N-terminal) and membrane proximal (C-terminal) sites to release mature TGF-alpha. Tumor necrosis factor-alpha converting enzyme (TACE) was shown to have a central role in the shedding of both factors. Here we show that cleavage of the proTGF-alpha C-terminal site, required for release of mature growth factor, is less sensitive to a panel of hydroxamates than TNF-alpha processing. Recombinant TACE cleaves TNF-alpha and N-terminal TGF-alpha peptides 50-fold more efficiently than the C-terminal TGF-alpha peptide. Moreover, fractionation of rat liver epithelial cell membranes yields two populations: one contains TACE and cleaves peptides corresponding to TNF-alpha and both proTGF-alpha processing sites, while the other lacks detectable TACE and cleaves only the C-terminal proTGF-alpha processing site. Activities in both fractions are inhibited by hydroxamates and EDTA but not by cysteine, aspartate, or serine protease inhibitors. Both membrane fractions also contain ADAM 10. ADAM 10 correctly cleaves peptides and a soluble form of precursor TGF-alpha (proTGFecto) at the N-terminal site but not the C-terminal site. However, the kinetics of N-terminal peptide cleavage by ADAM 10 are 90-fold less efficient than TACE. Our findings indicate that while TACE is an efficient proTGF-alpha N-terminal convertase, a different activity, distinguishable from TACE, exists that can process proTGF-alpha at the C-terminal site. A model that accounts for these findings and the requirement for TACE in TGF-alpha shedding is proposed.

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.

The role of transsignalling via the agonistic soluble IL-6 receptor in human diseases

The activation of cells that do not express the membrane bound interleukin-6 6 receptor (IL-6R) by IL-6 and the soluble IL-6 receptor (sIL-6R) is termed transsignalling. Transsignalling may be an pathogenetic factor in human diseases as diverse as multiple myeloma (MM), Castleman's disease, prostate carcinoma, Crohn's disease, systemic sclerosis, Still's disease, osteoporosis and cardiovascular diseases. IL-6 and sIL-6R may directly or indirectly enhance their own production on endothelial or bone marrow stromal cells. Positive feedback autocrine loops thus created in affected organs may either cause or maintain disease progression. In autoimmune or vasculitic disease, the ability of the IL-6/sIL-6R complex to inhibit apoptosis of autoreactive T-cells may be central to the development of tissue specific autoimmunity. The anti-apoptotic effect of the IL-6/sIL-6R complex may be involved in tumour genesis and resistance to chemotherapy. Only in rare cases, where counterregulation has failed, there is a notable systemic effect of IL-6/sIL-6R. Appropriate animal models are necessary to establish the pathogenetic role of the IL-6/sIL-6R complex. A specific treatment option for diseases influenced by the sIL-6R could be based on gp130-Fc, a soluble gp130 (sgp130) linked to the Fc-fragment of IgG1. gp130-Fc has shown efficacy in vivo in animal models of Crohn's disease.

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.

Transmodulation of cell surface regulatory molecules via ectodomain shedding

Cell responses to exogenous stimuli often result in a rapid decrease of cell surface density of a wide range of diverse regulatory proteins, receptor and adhesion molecules in particular. This decrease may occur in a ligand-dependent fashion (down-regulation), following endocytosis and degradation by lysosomal proteases, or by down-modulation, where molecules are targeted by endoproteases directly on cell surface. These proteases are recruited by trans-modulating agents, different from ligand, which act via their own receptors and the related intracellularly-generated signals. Endoproteolytic activity determines the release of large portions (shedding) of substrate proteins, called ectodomains, which are usually not ligand-bound, and therefore represent biologically-active molecules. Ectodomain shedding is involved in a number of pathophysiological processes, such as inflammation, cell degeneration and apoptosis, and oncogenesis. Common features of the process, such as the involvement of protein kinase C and of transmembrane metalloproteases, have been identified. In this review, we summarize basic concepts on down-modulation and ectodomain shedding, and provide an update of the issue with respect to: (i) new entries to the list of molecules found involved in the process; (ii) current views about the upstream control of shedding, i.e. the pathways linking the signals triggered by the trans-modulating agents to the activation of endoproteolytic activity on the cell surface.

Protein kinase C delta and eta isoenzymes control the shedding of the interleukin 6 receptor alpha in myeloma cells

The soluble interleukin 6 receptor alpha is an agonistic molecule of interleukin 6 (IL-6) and is important in the biology of multiple myeloma. More precisely, it potentiates the deleterious effects of IL-6 during tumour progression, facilitating angiogenesis and bone resorption. Because the mechanisms involved in the shedding of the interleukin 6 receptor alpha (IL-6Ralpha) in multiple myeloma are not known, we have investigated them in the XG-6 human myeloma cell line. Here we provide evidence that PMA-induced IL-6Ralpha shedding is controlled by a metalloproteinase and by protein kinase C (PKC) isoenzymes that do not require Ca(2+) for their activation. We show that XG-6 cells express PKC-delta, -eta and -zeta isoenzymes. However, after stimulation with PMA, only PKC-delta and PKC-eta are activated, as shown by their translocation to the membrane. Treatment with PMA induces an increase in PKC-delta phosphorylation in its active loop. In addition, by using rottlerin, a specific inhibitor of PKC-delta, we demonstrate that PKC-delta is involved in the PMA-induced shedding of IL-6Ralpha. With the use of UO126, a specific inhibitor of the mitogen-activated protein kinase (MAPK) pathway, we show that the PMA-induced IL-6Ralpha shedding is mediated in part by the MAPK pathway. Finally, whereas GF109203X, a general PKC inhibitor, inhibits the activation of ERK1/2 (extracellular signal-regulated protein kinase 1/2), rottlerin has no inhibitory effect, indicating that the Ras/MAPK activation is PKC-dependent but PKC-delta-independent. Taken together, these results suggest that the PMA-induced shedding of IL-6Ralpha is mediated by a PKC isoenzyme network.

Synergistic effects of glycoprotein 130 binding cytokines in combination with interleukin-1 on cartilage collagen breakdown

OBJECTIVE

To determine whether other glycoprotein 130 (gp130) binding cytokines can mimic the effects of oncostatin M (OSM) in acting synergistically with interleukin-1alpha (IL-1alpha) to induce cartilage collagen breakdown and collagenase expression, and to determine which receptors mediate these effects.

METHODS

The release of collagen and proteoglycan was assessed in bovine and human cartilage explant cultures. Messenger RNA (mRNA) and protein production from immortalized human chondrocytes (T/C28a4) was analyzed by Northern blotting and specific enzyme-linked immunosorbent assays. Collagenase activity was measured by bioassay. Cell surface receptors were detected by flow cytometry.

RESULTS

OSM in combination with IL-1alpha caused a rapid synergistic induction of matrix metalloproteinase 1 mRNA, which was sustained over a 72-hour period. Flow cytometric analyses detected both the OSM-specific receptor and the gp130 receptor at the chondrocyte cell surface, but failed to detect the leukemia inhibitory factor receptor (LIFR). Cartilage degradation assays revealed that, of the gp130 binding cytokines, only OSM and IL-6, in the presence of its soluble receptor (sIL-6R), were able to act synergistically with IL-1alpha to promote collagen release.

CONCLUSION

This study demonstrates that IL-6 can mimic OSM in synergizing with IL-1alpha to induce chondrocyte-mediated cartilage collagen breakdown and collagenase production. In order to have this effect, IL-6 requires the presence of its soluble receptor. The apparent absence of LIFR explains why other gp130 binding cytokines do not act in synergy with IL-1alpha. Since OSM, IL-6, and sIL-6R levels have all been shown to be elevated in the rheumatoid joint, our findings suggest that these cytokines may be key mediators of cartilage collagen catabolism in the inflammatory arthritides.

A soluble form of CD83 is released from activated dendritic cells and B lymphocytes, and is detectable in normal human sera

CD83 is an inducible glycoprotein expressed predominantly by dendritic cells (DC) and B lymphocytes. Expression of membrane CD83 (mCD83) is widely used as a marker of differentiated/activated DC but its function and ligand(s) are presently unknown. We report the existence of a soluble form of CD83 (sCD83). Using both a sCD83-specific ELISA and Western blotting, we could demonstrate the release of sCD83 by mCD83(+) B cell and Hodgkin's disease-derived cell lines, but not mCD83(-) cells. Inhibition of de novo protein synthesis did not affect the release of sCD83 during short-term (2 h) culture of cell lines although mCD83 expression was significantly reduced, suggesting sCD83 is generated by the release of mCD83. Isolated tonsillar B lymphocytes and monocyte-derived DC, which are mCD83(low), released only low levels of sCD83 during culture. However, the differentiation/activation of these populations both up-regulated mCD83 and increased sCD83 release significantly. Analysis of sera from normal donors demonstrated the presence of low levels (121 +/- 3.6 pg/ml) of circulating sCD83. Further studies utilizing purified sCD83 and the analysis of sCD83 levels in disease may provide clues to the function and ligand(s) of CD83.

Il-6 and its soluble receptor orchestrate a temporal switch in the pattern of leukocyte recruitment seen during acute inflammation

During acute inflammation, leukocyte recruitment is characterized by an initial infiltration of neutrophils, which are later replaced by a more sustained population of mononuclear cells. Based on both clinical and experimental evidence, we present a role for IL-6 and its soluble receptor (sIL-6R) in controlling this pattern of leukocyte recruitment during peritoneal inflammation. Liberation of sIL-6R from the initial neutrophil infiltrate acts as a regulator of CXC and CC chemokine expression, which contributes to a suppression of neutrophil recruitment and the concurrent attraction of mononuclear leukocytes. Soluble IL-6R-mediated signaling is therefore an important intermediary in the resolution of inflammation and supports transition between the early predominantly neutrophilic stage of an infection and the more sustained mononuclear cell influx.

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.

Role of different dialysis membranes in the release of interleukin-6-soluble receptor in uremic patients

BACKGROUND

Interleukin-6 (IL-6) exerts its actions through a cell-surface receptor system that consists of two transmembrane subunits: the IL-6 binding glycoprotein gp 80 (IL-6R) and the signal-transducing component (gp 130). Soluble forms of the IL-6R (sIL-6R) are generated by shedding of the membrane-associated proteins. The sIL-6R binds the ligand IL-6 with comparable affinity as the membrane-associated IL-6R and enhances the actions of IL-6.

METHODS

Our aim was to evaluate the role of both uremia and different dialysis membranes on peripheral blood mononuclear cell (PBMC) release (either in absence or in presence of mitogen stimulation) and plasma levels of sIL-6R. Ten patients chronically dialyzed with cuprophan membranes (CU), eight patients on regular dialysis treatment with polymethylmethacrylate (PMMA) membranes, 11 uremic nondialyzed patients (UR), and 12 healthy subjects (CON) were included in the study.

RESULTS

PBMCs harvested from CU spontaneously released significantly (P < 0.01) greater amounts of sIL-6R (881.8 +/- 80.1 pg/mL), as compared with CON (267.5 +/- 26.5 pg/mL), UR (258.4 +/- 38.1 pg/mL), and PMMA (288.4 +/- 24.6 pg/mL). Under mitogenic stimulation, the sIL-6R release was significantly (P < 0.01) increased in all groups. The greater PBMC production of sIL-6R in CU was followed by significantly (P < 0.01) higher levels of circulating soluble receptors (48.7 +/- 2.5 ng/mL, 60%), as compared with CON (30.5 +/- 1.9 ng/mL). UR also showed high circulating levels of sIL-6R (53.3 +/- 5.9 ng/mL), probably secondary to an impaired urinary excretion. Circulating levels of sIL-6R in PMMA were comparable to CON (30.3 +/- 3.3 ng/mL). Either the absence of monocyte activation or the adsorption of sIL-6R on the hydrophobic PMMA surface could explain this finding.

CONCLUSIONS

These results suggest an important role for poor dialysis biocompatibility of CU on the release of sIL-6R, which increases sIL-6R plasma levels, thereby enhancing the inflammatory effects of IL-6.

Production of Soluble Granulocyte Colony-Stimulating Factor Receptors from Myelomonocytic Cells

It has been speculated that a soluble form of G-CSFR might be physiologically present in humans, since G-CSFR mRNA that lacks a transmembrane domain has been identified from a human myelomonocytic cell line. Here, we demonstrate human soluble G-CSFR (sG-CSFR) of two different molecular sizes (80 and 85 kDa) on an immunoblot analysis using Abs generated against the amino-terminal, extracellular domain of the full-length G-CSFR. Both isoforms of sG-CSFR were able to bind recombinant human G-CSF (rhG-CSF). RT-PCR analysis with primers targeted outside of the transmenbrane region revealed that membrane-anchored G-CSFR is expressed at all maturation stages of purified myeloid cells, including CD34+CD13+ cells (blasts), CD11b−CD15+ cells (promyelocytes or myelocytes), CD11b+CD15+ cells (metamyelocytes and mature neutrophils), and CD14+ cells (monocytes). On the other hand, sG-CSFR mRNA was detectable in CD11b−CD15+, CD11b+CD15+, and CD14+ cells, but not in the CD34+CD13+ blast population. The serum concentration of both isoforms of sG-CSFR appeared to be correlated with the numbers of neutrophils/monocytes before and after rhG-CSF treatment in normal individuals. Thus, two isoforms of sG-CSFR are physiologically secreted from relatively mature myeloid cells and might play an important role in myelopoiesis through their binding to serum G-CSF.