Functional Expression of Soluble Human Interleukin-11 (IL-11) Receptor α and Stoichiometry of in Vitro IL-11 Receptor Complexes with gp130

Abstract

The interleukin-6 (IL-6) family of cytokines activates signaling through the formation of either gp130 homodimers, as for IL-6, or gp130-leukemia inhibitory factor receptor (LIFR) heterodimers as for ciliary neurotrophic factor (CNTF), leukemia inhibitory factor, oncostatin M , and cardiotrophin-1. Recent in vitro studies with IL-6 and CNTF have demonstrated that higher order hexameric receptor complexes are assembled in which signaling chain dimerization is accompanied by the dimerization of both the cytokine molecule and its specific receptor ␣ subunits (IL-6R␣ or CNTFR␣, respectively). IL-11 is a member of the IL-6 family and known to require gp130 but not LIFR for signaling. In this study we investigate the functional and biochemical composition of the IL-11 receptor complex. The human IL-11 receptor ␣-chain was cloned from a human bone marrow cDNA library. IL-11R␣ was shown to confer IL-11 responsiveness to human hepatoma cells either by cDNA transfection or by adding a soluble form of the receptor (sIL11R␣) expressed in the baculovirus system to the culture medium. In vitro immunoprecipitation experiments showed that sIL11R␣ specifically binds IL-11 and that binding is enhanced by gp130. Similarly to IL-6 and CNTF, gp130 is able to induce dimerization of the IL-11•IL-11R␣ subcomplex, the result of which is the formation of a pentameric receptor complex. However, in contrast to the other two cytokines, IL-11 was unable to induce either gp130 homodimerization or gp130/LIFR heterodimerization. These results strongly suggest that an as yet unidentified receptor ␤-chain is involved in IL-11 signaling.

Interleukin-11 Signals through the Formation of a Hexameric Receptor Complex

“…These results suggest that IL-11 mediates signal transduction through gp130 homodimerization, but published data regarding the composition and stoichiometry of the ternary IL-11 receptor complex has as yet not been conclusive. From immunoprecipitation experiments, similar to those reported here, Neddermann et al (40) described a pentameric receptor complex consisting of two IL-11, two IL-11R, and one gp130. IL-6, on the other hand, has been shown to form a hexameric receptor complex, consisting of two molecules each of IL-6, IL-6R, and gp130 (37,38).…”

“…8), although we cannot formally rule out the possibility of larger complexes. The fact that wild type IL-11 was shown to induce gp130 homodimerization, in the presence of IL-11R, contradicts previous published results (40). The formation of a ternary receptor complex containing at least two molecules of gp130, observed here, excludes the requirement for a novel signaling receptor component as proposed by Neddermann et al (40).…”

Interleukin‐6: Structure‐function relationships

“…From the similarities of the IL-1 1R with the nonsignaling ILdR and CNTFR (Hilton et al, 1994;Chkrel et al, 1995;Baumann et al, 1996b), it may be deduced that the ternary receptor-complex of IL-11 is hexameric. However, immunoprecipitation studies with IL-11, the sIL-11R, and sgp 130 failed to detect the formation of a hexameric complex (Neddermann et al, 1996). Instead, two IL-1 l/sIL-IIR complexes were found to form an intermediate pentameric complex with one gp130 molecule, suggesting that IL-1 1-induced intracellular signaling involves dimerization of gp130 with an as yet unidentified receptor (Neddermann et al, 1996).…”

“…However, immunoprecipitation studies with IL-11, the sIL-11R, and sgp 130 failed to detect the formation of a hexameric complex (Neddermann et al, 1996). Instead, two IL-1 l/sIL-IIR complexes were found to form an intermediate pentameric complex with one gp130 molecule, suggesting that IL-1 1-induced intracellular signaling involves dimerization of gp130 with an as yet unidentified receptor (Neddermann et al, 1996). In the putative hexameric IL-11 receptor-complex, site I on each IL-11 molecule would bind to one IL-1 1R and, similar to CNTF , the sole gp130 molecule would interact with site II on one IL-11 molecule (forming a GH/GHR-like trimer).…”

“…Unexpectedly, it was also found that these gp130 dimers are covalently linked through an intermolecular disulfide bridge (Davis et al, 1993b;Murakami et al, 1993). Similarly, disulfide-linked heterodimers of gp130 and the LIFR are formed upon activation of target cells with LIF or CNTF (Davis et al, 1993b), while OSM (Gearing et al, 1992), CT-1 (Pennica et al, 1995), and possibly IL-I 1 (Neddermann et al, 1996) also signal via heterodimerization of gp130. The activation of gp130 (and LIFR) by the IL-6-related cytokines thus conforms to the model of ligand-induced dimerization, reminiscent of the mechanism proposed for the activation of tyrosine kinase receptors (Ullrich & Schlessinger, 1990) (Fig.…”