Encyclopedia of Signaling Molecules

2018 Edition
| Editors: Sangdun Choi

IL6RA, Interleukin-6 Receptor Subunit Alpha

Reference work entry
DOI: https://doi.org/10.1007/978-3-319-67199-4_101661

Synonyms

Historical Background

The interleukin-6 receptor (IL-6R) is a type I transmembrane protein that was cloned and described in 1988 (Yamasaki et al. 1988). It serves as the non-signaling alpha-receptor of the multifunctional cytokine IL-6 and is expressed in a cell-type-specific manner. The IL-6R shares many features with other receptors of the IL-6 family of cytokines, notably the interleukin-11 receptor (IL-11R) and the ciliary neurotrophic factor receptor (CNTFR) (Garbers et al. 2012). All these cytokines signal through the common β-receptor glycoprotein 130 (gp130).

Domain Organization and Cellular Expression of the IL-6R

The IL-6R is a prototypical type I transmembrane protein that consists of three extracellular domains. The Ig-like domain (often referred to as “D1” domain) is followed by two fibronectin type III-like domains (“D2” and “D3”), which form the so-called cytokine-binding module that is mandatory for ligand binding and subsequent activation of the signal-transducing receptor complex. The three domains are followed by a so-called stalk region, whose length is critical for the initiation of signaling and thought to act as a “spacer” that positions the domains D1–D3 in the correct distance to the signal-transducing gp130 β-receptors (Baran et al. 2013). The stalk region is followed by a single transmembrane and an intracellular region, which do not participate in signal transduction (Yamasaki et al. 1988) (Fig. 1).
IL6RA, Interleukin-6 Receptor Subunit Alpha, Fig. 1

Domain structure of the interleukin-6 receptor. The Ig-like domain (D1) is followed by two fibronectin type III-like domains (D2 and D3), which comprise the cytokine-binding module (CBM). The stalk region (S) connects the three extracellular domains with the transmembrane region (TM) and the intracellular region (ICD)

The IL-6R is only expressed on very few cell types in the human body, which renders all other human cells irresponsive toward activation by IL-6 alone. The IL-6R is expressed at high amounts in the liver on hepatocytes. Activation of hepatocytes by IL-6, which is expressed, e.g., in the course of a bacterial infection, induces the so-called acute-phase response, leading to the secretion of acute-phase proteins. This group of proteins comprises different members, among them C-reactive protein, serum amyloid A, haptoglobin, alpha-2-macroglobulin, and many others, and is important in terms of resolution of the initial inflammation. Therefore, activation of the hepatic acute-phase response is considered a major immunomodulatory action of IL-6, which is mediated by the membrane-bound IL-6R receptor.

Besides hepatocytes, the IL-6R is expressed on different leukocyte subsets, e.g., CD4+ T cells, monocytes, macrophages, neutrophils, and megakaryocytes. Given this expression profile, it is not surprising that IL-6 plays a pivotal role in regulating the balance between Th17 cells and regulatory T cells (Treg). IL-6 in combination with TGFβ induces the conversion of naïve T cells to Th17 cells and concomitantly inhibits Treg differentiation, processes that are associated with a variety of inflammatory diseases, which are driven by Th17-dependent mechanism (Hunter and Jones 2015).

Although the cellular expression profile of the IL-6R is remarkably stable, different mechanisms have been described that regulate IL-6R expression (reviewed in (Wolf et al. 2014)). For instance, glucocorticoid treatment increases IL-6R mRNA and protein levels, thereby enhancing the cellular responsiveness toward IL-6 activation. Delta-1, a ligand of the Notch receptor, has been described to reduce IL-6R levels (Csaszar et al. 2014), whereas stimulation of cells with epidermal growth factor (EGF) induces cellular IL-6R levels in a mammalian target of rapamycin (mTOR)-dependent manner (Garbers et al. 2013).

Signal Transduction via the IL-6R

In order to activate cells, IL-6 has to bind initially to the IL-6R on its target cell. The IL-6R does not directly participate in the activation of the signaling cascades, but the formation of the IL-6/IL-6R complex is a prerequisite for the recruitment of two molecules of the signal-transducing β-receptor glycoprotein 130 (gp130) (Lütticken et al. 1994). In contrast to the IL-6R, gp130 is expressed ubiquitously, illustrating that expression of the IL-6R determines which cells can directly be activated by IL-6. Signaling via the membrane-bound IL-6R has been termed “classic signaling” (Fig. 2).
IL6RA, Interleukin-6 Receptor Subunit Alpha, Fig. 2

IL-6 signaling via the membrane-bound and the soluble IL-6R. Binding of IL-6 to cells expressing the membrane-bound IL-6R is referred to as classic signaling (left), whereas signaling of IL-6 via soluble forms of the IL-6R is called trans-signaling (right). Both modes of signaling activate a homodimer of the signal-transducing β-receptor gp130

After formation of the IL-6/IL-6R/gp130 complex, signal transduction is achieved by activation of the Janus kinase/signal transducer and activator of transcription (Jak/STAT), mitogen-activated protein kinase (MAPK), and the phosphatidylinositol-4,5-bisphosphate 3-kinase (PI3K) signaling pathways. The most important function has been attributed to the activation of the Jak/STAT pathway, which is triggered by the kinase Jak1 that initially phosphorylates specific tyrosine residues within the intracellular domain of gp130 (Rodig et al. 1998). These phosphorylated tyrosine residues serve as docking sites for STAT molecules, which are subsequently phosphorylated by Jak1. As the intracellular region of the IL-6R is dispensable for the initiation of IL-6 signaling, gp130-mediated signaling can also be initiated by soluble forms of the IL-6R (sIL-6R; see next paragraph). This second branch of IL-6 signaling has been termed “trans-signaling” (Fig. 2). Whether there are qualitative or quantitative differences between signaling via membrane-bound and soluble IL-6R has not been analyzed in detail.

Mechanisms of Soluble IL-6R Generation

Besides the membrane-bound IL-6R, several soluble forms of the IL-6R (sIL-6R) exist, which are found in human serum at concentrations around 30–70 ng/ml (Chalaris et al. 2011). Importantly, IL-6 binds to membrane-bound and soluble forms of the IL-6R with similar affinity, and the sIL-6R in complex with IL-6 acts as an agonist on cells expressing gp130.

Two mechanisms have been reported that lead to the generation of sIL-6R: differential mRNA splicing and proteolytic cleavage of the membrane-bound receptor. Differential splicing of the IL6R mRNA leads to the excision of the exon encoding the transmembrane region, and the splicing of the adjacent exons results in a frameshift and a premature stop codon, generating ten unique amino-acid residues at the C-terminus of this sIL-6R form that are not found in the membrane-bound variant. Differential splicing of the IL6R mRNA has been described to occur in different cell lines and is thought to account for 10–20% of the sIL-6R in human serum.

The majority of sIL-6R, however, originates from proteolytic cleavage of the protein at the cell surface by proteases, a process that is also known as ectodomain shedding. Several different proteases have been described to be capable of IL-6R cleavage, among them bacterial metalloproteases and serine proteases from human neutrophils. The best studied and understood, however, is cleavage by the two members of the a-disintegrin and metalloproteinase (ADAM) family of metalloproteases, ADAM10 and ADAM17. ADAM10 is regarded as the “slow” protease that constitutively cleaves the IL-6R from the cell surface and could thereby contribute to the steady-state sIL-6R levels in the human circulation. However, recent studies have shown that ADAM10 can also be activated by distinct stimuli like activation of the purinergic channel P2X7 or the ionophore ionomycin, thereby acting as an inducible sheddase of the IL-6R (Garbers et al. 2011; Lokau et al. 2016). Similarly, ADAM17 can be activated by a variety of stimuli, e.g., small chemical compounds like the phorbol ester phorbol-12-myristate 13-acetate (PMA) or the antibiotic anisomycin or cellular processes like apoptosis, depletion of cholesterol, or via activation of G-protein-coupled receptors (Chalaris et al. 2011). Irrespective of the stimulus, both proteases cleave the IL-6R in close proximity to the plasma membrane, and the cleavage site of ADAM17 has been mapped to Gln-357/Asp-358 (Müllberg et al. 1994). However, recent data point to a cleavage site two amino-acid residues further upstream between Pro-355 and Val-356 (Riethmueller et al. 2016). The cleavage site used by ADAM10 has not been determined so far, but published data suggest that either ADAM10 uses a different cleavage site or that it is able to cleave the IL-6R at multiple positions within the stalk region adjacent to the plasma membrane (Baran et al. 2013). To date, the cleavage site used in vivo has not been determined, and it is unclear which protease is responsible for the generation of the sIL-6R levels found in human serum.

Although these serum levels are remarkably stable and comparable among human individuals, a nonsynonymous coding single nucleotide polymorphism (SNP) within the IL6R gene (rs2228145), which leads to the exchange of Asp-358 to Ala-358, is a major genetic determinant of sIL-6R serum levels. Homozygous carriers of rs2228145 show a nearly twofold increase in sIL-6R serum levels (Rafiq et al. 2007). Although the differentially spliced sIL-6R is increased in these individuals (Stephens et al. 2012), the exchange of a single amino-acid residue adjacent to the cleavage site increases proteolytic conversion rates of the IL-6R by both ADAM10 and ADAM17 (Garbers et al. 2014).

The IL-6R as a Therapeutic Target

Given its important role in the course of inflammation, IL-6 and its receptor are attractive targets of therapeutic intervention. This finding led to the development of a humanized monoclonal antibody, termed tocilizumab, which binds to the IL-6R and prevents interaction of IL-6 with the cytokine-binding module within the IL-6R ectodomain, thereby blocking IL-6-induced signal transduction. Tocilizumab has been approved in more than 100 countries for the treatment of rheumatoid arthritis, and is (at least in some countries) also approved for the treatment of polyarticular juvenile idiopathic arthritis (PJIA), systemic juvenile idiopathic arthritis (SJIA), and Castleman’s disease.

Tocilizumab binds equally well to membrane-bound and soluble forms of the IL-6R and therefore efficiently blocks both classic and trans-signaling (Fig. 3). A designer protein, termed sgp130Fc, which consists of the six extracellular domains of gp130 fused to the Fc portion of a human IgG1 antibody, effectively binds to the complex of IL-6/sIL-6R. As it has no affinity to IL-6 alone, sgp130Fc specifically blocks trans-signaling of IL-6, leaving IL-6 classic signaling intact (Fig. 3). The specific blockade of IL-6 trans-signaling has been shown to be sufficient, sometimes even superior compared to the total inhibition of IL-6, to block inflammation in a variety of animal models (reviewed in (Jones et al. 2011; Scheller et al. 2014)). The sgp130Fc protein has already succesfully passed phase I clinical trials and currently undergoes phase II clinical trials.
IL6RA, Interleukin-6 Receptor Subunit Alpha, Fig. 3

Therapeutic inhibition of IL-6 signaling. Whereas the monoclonal antibody tocilizumab, which binds to the IL-6R, equally well blocks classic and trans-signaling of IL-6, sgp130Fc only binds to the sIL-6R/IL-6 complex and is therefore selective for trans-signaling

Summary

Since cloning of its cDNA and the identification of the protein that is responsible for binding of the cytokine IL-6 in 1988, the IL-6R has been a target of extensive research, which is still ongoing today. Description of its cellular localization, its expression pattern in the human body, and its role in the regulation of intracellular signaling pathways and the complex extracellular regulation by proteolytic cleavage have paved the way for the IL-6R to be recognized as an attractive therapeutic target that is used in the clinics.

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© Springer International Publishing AG 2018

Authors and Affiliations

  1. 1.Institute of BiochemistryKiel UniversityKielGermany