IL6RA, Interleukin-6 Receptor Subunit Alpha
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 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
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.
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.