SurveyInterleukin-6: Biology, signaling and strategies of blockade
Introduction
Cytokines are proteins, which are engaged in the communication between cells of the immune system. Furthermore, many cytokines perform regulatory functions outside of the immune system. The majority of cytokines show a four-helical protein fold [1]. Members of other protein families such as interleukin-1 (IL-1), IL-18, tumor necrosis factor α (TNFα) and tumor growth factor α (TGFα) are often also referred to as cytokines although they belong to different protein families, which are not discussed here. Evolutionary, cytokines and cytokine receptors are first found in insects such as drosophila where they are involved in the regulation of stem cell renewal [2].
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Interleukin-6
Interleukin-6 (IL-6) was molecularly cloned in 1986 as a B-cell stimulatory factor by the group of Kishimoto [3]. At this point it turned out that IL-6 was identical with several other factors being analyzed in several laboratories over the world. These factors included hepatocyte stimulating factor [4] and myeloid blood cell differentiation-inducing protein [5] indicating that IL-6 showed several activities outside of the immune system.
IL-6 is a 184 amino acid glycosylated protein, which can
IL-6 classic- and trans-signaling
The IL-6R can be proteolytically cleaved from the cell membrane thereby generating a soluble IL-6R (sIL-6R), which can still bind its ligand IL-6 [11]. In humans, a sIL-6R can also be generated by translation of an alternatively spliced mRNA [12] although this mechanism seems to be less important than proteolytic cleavage [13]. Since the cytoplasmic portion of the IL-6R is not signaling competent, it could be removed without loss of signaling [7]. Likewise, the transmembrane domain of the IL-6R
Viral Interleukin-6
The genome of Human Herpes Virus 8 (HHV8) encodes a protein, which shows 25% identity with human IL-6 [23]. This viral cytokine could not only stimulate cells expressing IL-6R and gp130. It turned out that viral IL-6 does not require the IL-6R for inducing dimerization and activation of gp130 [24]. The structure of viral IL-6 in complex with the extracellular portion of gp130 was solved and confirmed the direct binding of viral IL-6 to gp130 [25]. Interestingly, the loop between helix B and
The IL-6 buffer in the blood
The IL-6 concentrations of healthy individuals in the blood are in the range of 1 pg/ml [15]. In contrast, concentrations of sIL-6R and sgp130 are in the range of 50–75 ng/ml and 400 ng/ml, respectively. In the course of inflammatory states, the levels of sgp130 are largely maintained whereas concentrations of sIL-6R usually increase by a factor of 2–5 [29]. The concentrations of IL-6, however, increase up to 150 ng/ml in patients with autoimmune diseases such as Rheumatoid Arthritis [30]. During
Intracellular signal transduction of IL-6
Generally, IL-6-induced signaling can be inhibited either by targeting the extracellular or the intracellular part of IL-6 signaling. Targeting extracellular soluble proteins (e.g. IL-6 or sIL-6R) or extracellular domains of transmembrane proteins (e.g. IL-6R) is more specific and less difficult than targeting intracellular structures, such as kinases, due to their limited accessibility. Extracellular structures can be targeted with biologics, such as monoclonal antibodies (e.g. Tocilicumab,
Inflammatory action of IL-6
Engagement of Toll like receptors (TLR) leads to the activation of the NF-κB pathway, which is one of the strongest stimuli for the synthesis of IL-6 [145]. Furthermore, IL-1 is a strong stimulator of IL-6 synthesis in fibroblasts [146]. Since IL-1 is also induced upon stimulation of TLRs, IL-1 is part of a feed-forward loop which increases the production of IL-6 [146]. Besides its multiple roles in the innate immune system, IL-6 is necessary for the differentiation of TH17 cells [147]. As
Regenerative and protective action of IL-6
As it became already clear when the first IL-6 cDNA was cloned, the physiologic role of IL-6 is not restricted to the stimulation of the immune system [8]. It is known that IL-6 plays an important role during liver regeneration [150], is needed for the control of the body weight [151] and for the development of local IgA antibody responses [152]. In animal models of inflammatory bowel disease it became clear that IL-6 was required for the regeneration of intestinal epithelial cells and for the
Blockade of IL-6 for the treatment of autoimmune diseases
The humanized anti-IL-6R antibody Tocilizumab has been approved in more than 100 countries for the treatment of Rheumatoid Arthritis and other autoinflammatory diseases [158]. IL-6 blockade with Tocilizumab was shown to be at least as beneficial in patients with Rheumatoid Arthritis as blockade of TNFα [159] indicating that IL-6 plays a key role in the development and maintenance of autoimmune diseases.
Recently it became clear that pro-inflammatory activities of IL-6 are mainly mediated by the
Closing remarks
IL-6 can act by two different pathways, which employ the membrane bound and the soluble IL-6R. While classic signaling via the membrane bound IL-6R is considered mostly protective and regenerative, IL-6 trans-signaling can be considered to represent a stress response of the body to maintain body homeostasis. Since overshooting IL-6 activities during autoinflammatory conditions such as Rheumatoid Arthritis or inflammatory bowel disease are considered to be mainly mediated by IL-6
Conflict of interest statement
SRJ is the inventor of the sgp130Fc protein and an inventor on several patents owned by CONARIS Research Institute, which develops the sgp130Fc protein together with Ferring Pharmaceuticals, and he has stock ownership in CONARIS. He has acted as a consultant and speaker for Genentech Roche, Chugai, AbbVie and Janssen.
Acknowledgements
This work is supported in part by grants from the German Federal Ministry of Education and Research (BMBF): E:Bio-Modul II InTraSig to FS and SRJ. The work of FS was also supported by the German Federal Ministry of Education and Research (BMBF): E:Bio-Modul I JAK-Sys and by the Deutsche Forschungsgemeinschaft (SCHA 785/8-1). The work of SRJ was supported by the Deutsche Forschungsgemeinschaft Bonn, Germany (SFB841, project C3; SFB877, project A1 and Cluster of Excellence ‘Inflammation at
Fred Schaper received a Diploma in Biology in 1992, and the Dr. rer. nat. (Ph.D.) degree in 1996 from the Technical University Carolo Wilhelmina of Braunschweig, Germany. The practical parts of both studies have been realized at the German Research Centre for Biotechnology (GBF) Braunschweig, Germany. He became junior research group leader at the Department of Biochemistry and Molecular Biology at the RWTH University, Aachen, Germany in 1996, received the venia legendi for Biochemistry and
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Fred Schaper received a Diploma in Biology in 1992, and the Dr. rer. nat. (Ph.D.) degree in 1996 from the Technical University Carolo Wilhelmina of Braunschweig, Germany. The practical parts of both studies have been realized at the German Research Centre for Biotechnology (GBF) Braunschweig, Germany. He became junior research group leader at the Department of Biochemistry and Molecular Biology at the RWTH University, Aachen, Germany in 1996, received the venia legendi for Biochemistry and Molecular Biology in 2002 and became adjunct professor at the same location in 2005. Fred Schaper has been appointed to a full professor (W3) at the Department of Biology of the Otto-von-Guericke-University, Magdeburg, Germany and chairs the Department of Systems Biology since 2010. His interests are focused on the regulatory and dynamic aspects of IL-6 signaling and the cross-talk of IL-6 with other cytokines and hormones.
Stefan Rose-John obtained his Doctoral degree in Biological Sciences at the University of Heidelberg, Germany. He worked for two years as a postdoc in Michigan before he returned to Heidelberg to join the Institute of Biochemistry of the German Cancer Research Center in Heidelberg. He moved to the RWTH Aachen where in 1992 he obtained his Habilitation in Biochemistry. In 1994, he became C3-Professor at the University of Mainz. Since 2000, he is C4-Professor and Director at the Institute of Biochemistry of the University of Kiel in Northern Germany. Stefan Rose-John's laboratory is focused on understanding the molecular biology of cytokines. Stefan Rose-John has shown in the past years that Interleukin-6 ‘trans-signaling’ via the soluble Interleukin-6 receptor is important for the regulation of cellular differentiation and of apoptosis. He is currently developing a novel therapeutic concept for the treatment of chronic inflammatory diseases and cancer. Indeed, he developed a specific ‘trans-signaling’ antagonist, which – in animal models – has proven effective in blocking chronic inflammatory diseases such as Crohn's diseases, Rheumatoid Arthritis and inflammatory colon cancer. This antagonist has already been tested in Phase I clinic trials in 2013/14 and will enter Phase II clinic trials later in 2015.
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