title: Influence of interleukin-6 (IL-6) dimerization on formation of the high affinity hexameric IL-6.receptor complex.
abstract: The high affinity interleukin-6 (IL-6) signaling complex consists of IL-6 and two membrane-associated receptor components: a low affinity but specific IL-6 receptor and the affinity converter/signal transducing protein gp130. Monomeric (IL-6M) and dimeric (IL-6D) forms of Escherichia coli-derived human IL-6 and the extracellular ("soluble") portions of the IL-6 receptor (sIL-6R) and gp130 have been purified in order to investigate the effect of IL-6 dimerization on binding to the receptor complex. Although IL-6D has a higher binding affinity for immobilized sIL-6R, as determined by biosensor analysis employing surface plasmon resonance detection, IL-6M is more potent than IL-6D in a STAT3 phosphorylation assay. The difference in potency is significantly less pronounced when measured in the murine 7TD1 hybridoma growth factor assay and the human hepatoma HepG2 bioassay due to time-dependent dissociation at 37 degrees C of IL-6 dimers into active monomers. The increased binding affinity of IL-6D appears to be due to its ability to cross-link two sIL-6R molecules on the biosensor surface. Studies of the IL-6 ternary complex formation demonstrated that the reduced biological potency of IL-6D resulted from a decreased ability of the IL-6D (sIL-6R)2 complex to couple with the soluble portion of gp130. These data imply that IL-6-induced dimerization of sIL-6R is not the driving force in promoting formation of the hexameric (IL-6 IL-6R gp130)2 complex. A model is presented whereby the trimeric complex of IL-6R, gp130, and IL-6M forms before the functional hexamer. Due to its increased affinity for the IL-6R but its decreased ability to couple with gp130, we suggest that a stable IL-6 dimer may be an efficient IL-6 antagonist.
authors.affiliation: Joint Protein Structure Laboratory, Ludwig Institute for Cancer Research (Melbourne), Vicotoria, Australia.
authors.forename: L D
authors.initials: LD
authors.lastname: Ward
authors.forename: A
authors.initials: A
authors.lastname: Hammacher
authors.forename: G J
authors.initials: GJ
authors.lastname: Howlett
authors.forename: J M
authors.initials: JM
authors.lastname: Matthews
authors.forename: L
authors.initials: L
authors.lastname: Fabri
authors.forename: R L
authors.initials: RL
authors.lastname: Moritz
authors.forename: E C
authors.initials: EC
authors.lastname: Nice
authors.forename: J
authors.initials: J
authors.lastname: Weinstock
authors.forename: R J
authors.initials: RJ
authors.lastname: Simpson
mesh_terms: D015703:Antigens, CD / Q000737:chemistry* / Q000378:metabolism; D050822:Cytokine Receptor gp130; D004268:DNA-Binding Proteins / Q000378:metabolism; D006801:Humans; D015850:Interleukin-6 / Q000737:chemistry*; D007700:Kinetics; D046911:Macromolecular Substances; D008562:Membrane Glycoproteins / Q000378:metabolism; D008970:Molecular Weight; D019000:Phosphotyrosine / Q000378:metabolism; D011485:Protein Binding; D011940:Receptor Aggregation; D018123:Receptors, Interleukin / Q000737:chemistry*; D019947:Receptors, Interleukin-6; D011994:Recombinant Proteins; D050796:STAT3 Transcription Factor; D015398:Signal Transduction; D015534:Trans-Activators / Q000378:metabolism
chemical_list: D015703:Antigens, CD; D004268:DNA-Binding Proteins; C494199:IL6ST protein, human; D015850:Interleukin-6; D046911:Macromolecular Substances; D008562:Membrane Glycoproteins; D018123:Receptors, Interleukin; D019947:Receptors, Interleukin-6; D011994:Recombinant Proteins; D050796:STAT3 Transcription Factor; C494086:STAT3 protein, human; D015534:Trans-Activators; D050822:Cytokine Receptor gp130; D019000:Phosphotyrosine