IDENTIFICATION OF A REGULATORY DOMAIN OF THE INTERLEUKIN-6 RECEPTOR

IL-6 signal transduction occurs when the liganded interleukin-6 recept or (IL-6R) interacts with glycoprotein (gp) 130. We hypothesized that synthetic peptides modeled from the extramembranous domain of the IL-6 R may interfere with the IL-6-induced reaction between IL-6R and gp130 and may serve to elucidate the initial steps in IL-6 signal transduct ion. The capacity of such peptides to modulate two different IL-6 func tions was evaluated: 1) IL-6-dependent B9 cell mitogenesis, and 2) IL- 6-induced acute phase protein synthesis in HepG2 cells. A synthetic pe ptide, (249)Y16T(264), corresponding to residues 249-264, inhibited IL -6-dependent B9 proliferation and IL-6-induced acute phase protein up- regulation in HepG2 cells. Other peptides modeled from different regio ns of the IL-6R were not inhibitory. (249)Y16T(264) did not inhibit IL -6-independent HepG2 cell proliferation or total cellular protein synt hesis. The inhibitory effect was reversible, indicating that the pepti de was not cytotoxic. (249)Y16T(264) did not inhibit I-125-IL-6 bindin g in U266 cells. Delineation of this domain identified (249)Y10R(258) as the minimum effective sequence capable of inhibiting fibrinogen syn thesis. Amino acid substitutions in (249)Y10R(258) obliterated the inh ibitory effect on fibrinogen synthesis. In conclusion, a region of the extramembranous domain of the IL-6R has been identified that is invol ved in the regulation of IL-6 signal transmission. A synthetic peptide representing this region inhibits IL-6-dependent B9 cell mitogenesis and IL-6-stimulated acute phase response in HepG2 cells without affect ing ligand binding.