Evan T. Keller^1,2,3, Jon Wanagat¹, W.B. Ershler^1,3

¹Glennan Center for Geriatrics and Gerontology, Departments of ²Pathology and ³Internal Medicine, Eastern Virginia Medical School.

Received 10/21/96; Accepted 11/01/96; On-line //96

5. THE INTERLEUKIN-6 RECEPTOR

5.1. The interleukin-6 receptor: Structure and function

The human IL-6R (also known as gp80 and the IL-6Ra subunit), was first cloned by Yamasaki et al. from a human natural killer-like cell line, YT, (162) followed by Schooltink et al. from a human hepatoma cell line, HepG2 (163). IL-6R is a 80 Kd protein consisting of 467 aa. Located on chromosome 1 band q21 (164), the IL-6R gene encodes for a 5 Kb mRNA containing a coding region of 1401 bp (162). The 3'-untranslated region (3'-UTR) consists of approximately 1.5 Kb, suggesting that the 5'-UTR compromises approximately 2.1 Kb. The human IL-6R shares 53% identity with the rat hepatic IL-6R aa sequence (165). However, the mRNA structures are strikingly different. Although both human and rat IL-6 mRNAs are approximately 5 Kb, the rat 3'-UTR consists of approximately 3.1 Kb, which combined with the observation that the coding region, similar to the human IL-6R coding region, is 1.4 Kb, suggests that the 5'-UTR is much shorter in the rat than the human IL-6R mRNA (approximately 0.6 vs 2.1 Kb, respectively). A definitive conclusion on the length of the 5'-UTRs of these mRNAs awaits for localization of the transcription initiation site(s).

The structure of IL-6R has been deduced by comparative sequence analysis. A hydropathy plot revealed two major hydrophobic regions; one which encodes for the signal peptide between residues 1 and 20, and the other which encodes for the transmembrane domain in the region of residues 359 to 386 (162). The latter region is followed by a putative transmembrane anchoring stop codon consisting of several positively charged residues. These findings suggest the IL-6R has a 339 aa extracellular region, 28 aa transmembrane region, and 82 aa intracellular region. Intriguingly, the intracellular region does not contain any kinase domains suggesting that this molecule is not capable of signaling activity (162).

Upon homology search, it was identified that the extracellular component of the IL-6R contains a domain which shares extensive homology with the Ig superfamily (162) and two tandem fibronectin type III motifs (166) present in a 200 aa region. This region defines the cytokine receptor family domain, a domain which is found in a variety of other cytokine and growth factor receptors. It contains highly conserved components consisting of four cysteine residues in its amino-terminal region, and a tryptophan-serine-X-tryptophan-serine (WSXWS) motif penultimate of the transmembrane region (166, 167). Fibronectin type III domains are observed in cell-adhesion molecules, which implies that cytokine receptors evolved from an ancestral adhesive molecule (168).

As the protein structure suggests, the IL-6R is not capable of inducing signal transduction directly. It is now understood that in order for IL-6 to mediate signal, it first binds to gp-80 forming a low affinity receptor complex. This complex then associates with the non-ligand-binding transmembrane glycoprotein, gp-130 (169). Homodimerization of gp-130 is required for IL-6 signal transduction (170). Although it was originally considered that one unit of IL-6 and the IL-6R bound to a gp-130 homodimer (170), the stochiometry and number of this reaction appears to involve a hexameric complex consisting of two molecules each of IL-6, IL-6R gp-80, and gp-130 (171). This complex forms a high affinity binding site for IL-6, as opposed to the low affinity binding observed with IL-6 and IL-6R gp-80 in the absence of gp-130.

Mutational analyses of the IL-6R has identified that the region of amino acids 106-322, which comprise the cytokine receptor family domain of IL-6R, is responsible both for IL-6-binding and for binding to gp-130 (172). In fact, the Ig-like domain, whose action in the context of IL-6R is not currently identified, is not required for either of these functions.

The IL-6R has an isoform which was first identified in human urine, the soluble IL-6R (sIL-6R) (173). In contrast to other soluble cytokine receptors (e.g., sIL-2R) which inhibit cytokine induced signaling, the sIL-6R forms a fully active hexameric IL-6:sIL-6R:gp-130 complex which induces cell signaling. How sIL-6R is generated is not currently known , but both alternative splicing, resulting in loss of the transmembrane domain (174), or proteolyis of the mature cell surface IL-6R (175, 176) have been proposed.

As mentioned in our opening comments, IL-6 is one of a family of cytokines collectively termed "the interleukin-6-type cytokines". The cytokines which make up this family are IL-6, leukemia inhibitory factor (LIF), oncostatin-M (OSM), ciliary neurotrophic factor (CNTF), cardiotrophin-1 (CT-1), and interleukin-11 (177). The common characteristic which define this group is they activate gp-130 to induce cell signaling. That these cytokines converge on gp-130 underscores the redundancy present in the cytokine system. However, specificity of cytokine action is mediated, in part, through cell specific repertoire of cytokine specific a chain receptors, such as gp80 for IL-6

5.2. Expression of the Interleukin-6 Receptor

The IL-6R is expressed in a variety of cells (Table 3). In general it is expressed in the range of 100 to 2000 sites/cell (178). However, in myeloma lines and Epstein-Barr virus transformed lines up to 29,000 sites/cell have been identified (178). Except for the effects of dexamethasone, modulation of IL-6 expression by various factors has not given consistent results. Snyer et al. demonstrated that A23187 (a calcium ionophore), lipopolysaccharide, prostaglandin E1, IL-1, tumor necrosis factor (TNF), and muramyl dipeptide did not significantly alter IL-6R expression in either CESS (Epstein-Barr Virus B cell immortalized line), HL-60, U937, Hep-G, UAC cell lines. In contrast, several other groups have demonstrated that IL-1 does modulate IL-6R expression in several cell lines and tissues (summarized in Table 3). Perhaps cell specific differences in response account for the discordant results. On the other hand, dexamethasone has been consistently demonstrated to increase IL-6R in a variety of tissues including liver primary cells and cell lines, monocyte primary cultures, myeloma cell lines, and an amniotic cell line.

In spite of the great variety of cells which express the IL-6R and its importance in many facets of physiology, the molecular mechanisms which regulate transcriptional control of the IL-6R gene have not been defined to date. Cloning and analysis of the IL-6R promoter will help define these mechanisms.